TL;DR: No, there’s nothing to worry about. You can happily use restic now and for a long time from now.
I might sound biased, but I assure you that the following is said completely objectively; Restic is probably one of the absolutely best options you can find if backwards compatibility and being able to restore is important to you.
The reason for this is that the most important part of restic is the repository format, which itself is actually considered the public API in restic.
The repository format is very clearly documented specifically for the purpose of not locking anyone into something that cannot be read in the future. This was a very conscious design decision by @fd0 when he started creating restic many years ago
There has already been a few examples of people fiddling around and building software that can read/write from/to a restic repository, which shows that what is describe above works in practice.
Hence, as long as you make sure to keep a copy of the design document/reference, you should be able to create new software that reads that data from your repository, in the very unlikely event that there’s no other software around to do it. Of course you also need the password for the repository (which if you were to not care about that security aspect, you could just note in a text file along with the design document in your Glacier, e.g. if you don’t care about that and would otherwise just have synced files using rsync).
Stop worrying so much
This is a question that obviously no-one can answer because it’s about the future. If you fast-forward 200 years, who knows if your binary can still be run. Probably, in some kind of emulator.
On a more pragmatic note, there’s no reason to think that restic binaries cannot be run for a long time from today, and if there were to ever comes a time when restic binaries will stop working, it will not happen over night - you will have several years if not decades to adapt to this fact and find a solution that works.
If you want some kind of similarity to compare with, consider the migration from x86 to x64 architecture - we can still run 32 bit software even though we’ve had 64 bit architectures for I don’t know how long now… Things like this don’t just vanish over night.