You're probably correct, they've already delisted some DLC songs due to license expiry...

But if they are or will be relicensing many of the songs for their subscription service Rocksmith+, why couldn't they use those same licenses to keep distributing Rocksmith 2014 and its DLC in stores? Unfortunately I'm not too thorough with music licensing.

which is still bullshit

Even with open source software, there would still need to be license to use the music and that is expiring

the software being open source would do nothing to solve the underlying issue, which is expiring music rights

The one thing a subscription kinda makes sense for, as opposed to paying for every song a la carte, and they don't even have all the songs of the previous games. What a fucking embarrassment.

I imagine the licensing just doesn't carry forward and it has to be re-negotiated, but if that's the case then they're hilariously slow at it. I check every six months or so and there's still only a handful of bands I recognize.

Shouldn't the same RealTone cable work?

You can use the same cables. The Rocksmith cable is simply a standard guitar to USB, it isn't doing anything Linux doesn't know how to handle.

Any microphone or better yet audio interface with an "instrument in" jack will work with the game. I'd personally rather not rely on a single use type computerized cable from a game company that doesn't support the game anymore.

Yes. There is a mod that allows audiointerfaces to be connected directly to RS via asio drivers.

https://github.com/mdias/rs_asio

Tho it is stated that you also need wineasio for linux.

You need some fiddeling to get it working even under windows but once it works its almost perfect.

Generally it will work. If it's recognized as an audio input or microphone then it will work just fine under Microphone mode, if you want to use it like a RealTone Cable or whatever it was called, you will have to rename that audio input interface and make sure the bitrate and number of channels matches that cable.

Ubisoft has announced that people who have the game and official DLCs, will continue to be able to use them after delisting.

And as far as I can tell, the game still works despite not connecting to Ubisoft servers, so Ubisoft would have to release an update to the game purposely to break its offline function (now I wouldn't put it past Ubi to do it but unlikely).

Historically, and I believe per the announcement, any content you'd already bought you will be able to download even after it's been delisted.

If you have purchased RS2014 you will still be able to use it, and with the DLC installed you will still be able to play custom maps though I presume the amount of submissions will continue to dwindle.

No - it should remain in your library.