Until they hit a hard bug or don't support newer transport formats or scenarios. Also the community dries up eventually
That is why you test your backuo. It is unrealiatic, that in a stable software release there is suddenly, after you tested your backup a hard bug which prevents recovery.
Yes unmaintained software will not support new featueres.
I think you misunderstood me. You should not use unmaintained software as your backup tool, but IMO it is no problem when it suddenly goes unmaintained, your backup will most likely still work.
Same with any other software, that goes unmaintained, look for an alternative.
I agree with you here. It is similar to "my phone is listening to what i am talking about and it shows me topic related ads" which has been basically disproven many times over the years.
One of the main reasons why I avoid softwares such as Kopia or Borg or Restic or whatever is in fashion:
they go unmantained
they are not simple: so many of my frienda struggled restoring backups because you are not dealing with files anymore, but encrypted or compressed blobs
rsync has an easy mental model and has extremely good defaults
Going unmaintained is a non issue, since you can still restore from your backup. It is not like a subscription or proprietary software which is no longer usable when you stop to pay for it or the company owning goes down.
The design of restic is quite simple and easy to understand. The original dev gave multiple talks about it, quite interesting.
Imho the additional features of dedup, encryption and versioning outweigh the points you mentioned by far.
With the right timing/deals you get them even cheaper.
Mindfactory had 20 TB Seagate exo drives on a deal for 219€ (~11€/TB)
The 18 TB Seagate exos were often on 239€. (13.2€/TB)
I use full disk encryption for every server (and other computers).
Encrypting your data drives is a must for everyone imho. Encrypting the OS is a must for me🤷♂️