If you are going to inherit something, it's certainly a better position that they were using something open like Home Assistant than if they were using a bunch of proprietary / cloud based things.

At least you have the capability of getting things into a working state without the original owner, which may be impossible otherwise.

Now of course the best case scenario is when the "smartness" is only additive to the home, and not required for it.

In my case, if Home Assistant isn't working, the house just works like a normal house with all the wall switches functining as you would expect.

If I ever sold the house, I would give the new owner the Home Assistant instance, disable all my automations, but also tell them that I'm not going to help them at all with supporting it. The most likely scenario is that they wouldn't want to use it or care about it, but at least the house works just fine without it.