What if we cannot afford the space of keeping everything backed uo forever?

You enforce a reasonable data retention policy, or charge for it.

What if it has been a year? Where do we put the limits to “okay, this is stupid” and “this is perfectably reasonable”?

If you fail to recover data for everyone, then the data retention is too low. If you succeed to recover data for everyone, then the data retention is too high. Pick a data retention policy that leans towards long enough that you can recover data for most people, or charge extra for it. It's not that complicated.

What if the action cannot be reversed,

Tech support can reverse the action in this case, so I don't see how this is relevant.

[...], and after deletion you need to anonimyze particularly sensitive data?

Most software doesn't process credit card transactions, so I don't see how this is relevant. Even if they did, they probably have to keep the data around due to regulatory requirements.

I say to all that, READ THE FUCKING MANUAL. If you are not apt enough to read and research about the software, you are not apt enough to use it.

People should at least try to make usable software first, but manuals are fine.

Same with hardware. You cut your finger because you didnt follow instructions clearly laid out for you not to cut your finger when using a saw? Maybe sawing was not for you mate

Yeah, shit happens, assuming they receive proper training and the saw complies with safety standards.