150MB per 10min video, and that means it's 390,000Mb (or 380.86TB) for their collection.

Your overall point is fair, but your math here is off by a factor of 1000 - it would be around 380 GB.

It could be done if peertube used a scheme like BitTorrent. We are approaching a time where enough users have sufficient upstream bandwidth for video.

But then, even without hosting costs, creating videos takes much more time and effort than writing a short text.

You are vastly overestimating the amount of storage you need since you are looking at some download which itself has to choose the encoding (which is independent of whatever youtube does: youtube absolutely crushes the quality).
Most estimates assume that youtube has 1 exabyte of storage, let's say we buy this in bulk from retail (which we wouldn't do: you wait as long as possible since storage prices are going down and retail stores would give you the finger if you ordered and exabyte worth).
Let's take that number and run with it:
Buying retail, you can get Seagate Exos X20 20TB drives for 280€, 1 exabyte is 1Mio terabyte, meaning we have 1_000_000/20 * 280 = 14 Mio € (you'd need machines to put those into but you also wouldn't buy the entire thing upfront, and using retail prices either).

Compute also isn't that big of a deal if you do it correctly: the expensive part in video hosting is usually video encoding since to get small video sizes you need to spend compute beforehand to compress it.
However, you can shift this in significant parts to the user by implementing the transcoding in WASM and running this clientside (see e.g. https://www.w3.org/2021/03/media-production-workshop/talks/qiang-fu-video-transcoding.html) in that case users would compress locally in the browser before uploading (this presumably wouldn't even take longer than normal uploads for most people since you trade off transcoding time against upload time).
There are still other compute expenses but those are much more limited.
These mechanisms don't (at least to my knowledge) exist in peertube yet, but would be possible.

The actually expensive part is always the actual networking: Networking is one of the few things that actually get more expensive at scale due to the complexity explosion, rather than cheaper (e.g. having dedicated transcoding hardware drops in price per user since you have higher utilization).
Networking quickly runs into bottlenecks where you have to account for all the covariances between datasets in the network.
Basically to increase the amount of e.g. storage available everything in the network needs to be increased (from the local machines connections, over the cables and switches up to routers and outgoing connections) due to you increasing the density at one point, you have to increase the network everywhere.
That's why networking dwarfs everything: you just get crushed by networking being the bottleneck between your increasingly dense devices.

The clue behind peertube is that this is not as extreme of an effect due to

1. federation (certain connections just aren't dense due to the overall network topology being distributed)
2. torrents

The latter is the important part: instead of having network cost rising (super) linearly to the amount of users you have it rise linearly to the amount of simultaneous unique videos.
This is a much smaller number which means you do not need to compete in that space, which is the dominant cost factor. (if you have a method where one user can retain the video and share it without actively watching that same video, you can probably get real-world sublinear scaling)

Mind you, the costs involved here are still large, but not insurmountably large, especially considering there is not one unique organisation that would have to pay for the entire thing and its not an upfront expense. Fundamentally though the system is built such that it won't be crushed as users flood into the network.

That's why it needs to be an international project. Paid by every country together. Sure some will initially have to pay more but sooner or later everyone wants to be part of it and pay their part.

Yeah, storage and bandwidth are massive considerations and there's no way Peertube can handle it. And each channel running their own instance actually makes it worse, since you're going to have smaller entities who can't take advantage of deals that larger companies can make for hardware, data centers, bandwidth, etc. Plus, if you're having to run your own instance to have a channel, then you're not just focusing on creating videos for the channel, now you're also a system architect, sysadmin, etc. It makes it a massive barrier to entry, and one that only tech enthusiasts will even consider tackling.

But even say that happens: a bunch of people running their own instances for their channel. Where are they hosting it? Are they purchasing their own hardware? Running their own data centers? They're most certainly not running it out of their home. The overhead for that kind of operation is massive. What you'll end up with is a bunch of people running their instances on AWS or some other PaaS provider. And then you're right back to the problem you're trying to solve with a distributed service: that the service is consolidated on one platform (even if it doesn't appear that way to the end user). Sure, AWS et al aren't dictating the terms of service for your Peertube instance, but the instance is dependent on that platform.

On top of all that, you have the issue of monetization. How are you going to make money from your channel? Peertube doesn't have the kind of infrastructure of advertising etc. that YT has.

You also have another massive issue: legal. YT spent over a decade going through the courts with the MPAA, RIAA, et al fighting about copyright issues. Google has massive amounts of money and was able to weather that fight. But it's competitors didn't. Which is why you don't have Vimeo stars, for example.

Running a YT channel is a massive time, energy, and money sink. Add all of these other considerations to it, and it's an impossible task. It's hard to think someone would could see PT as a viable alternative. Google destroyed all of the competition (or let attrition do it for them), and pulled the ladder up behind them.

380 GB in storage for multiple years of contents is really not much. I archive that amount every 2 months.

The real problem is serving all that content to the viewers, and the first bottleneck is usually the upload bandwidth.
I think the more interesting number would be to know how much data would it be to upload an average sized video to every viewer of it.
Using your example of a 15 MB video, serving that to 300.000 viewers means uploading roughly 4,5 TB data, plus some for technical data (TCP/IP and HTTP headers and such). For every (average) video! Now that's a lot!

Fortunately PeerTube helps with that: viewers will automatically upload their downloaded chunks of the video to the others currently viewing it, so in the end the server needs somewhat less bandwidth usage.

Other than that, it would be the perfect place where channels could team up to host shared instances for themselves, or every channel their own one but with redundancy set up, so that their friend channels could also chip in with the bandwidth when needed.