If they gave more time than just a few months, in example at least an entire year, then people could at least download those files before hand. Not sure how long this would take and how stressful this would be for their servers.

They could also give the option to pay for this. They should know what storing a video costs and therefore could give creators the chance to cover that cost.

Peertube won't cut it. Those people in the article have thousands of hours of videostreams there. If you're streaming at 1080p, that will be around 1.5GB of storage per hour. 4k will be worse. So if you have 5000 hours of videos like the one guy in the article, that is a neat 7500GB or 7.5TB of video. There is no instance around that will allow you to save that amount of videos.

So hosting your own instance would be the only way. Looking at Hetzner storage box, 10TB of data will cost you 25€/month or 300€/years. That is money, but should be possible to pay out of your own pocket.

You should at least play around with the export options and do regular exports. You really don't want your diary of over 5 years to be stuck in an unmaintained app. And since this seems to be supporting exporting into markdown - maybe try an export and checkout logseq or Obsidian?