Your effective wi-fi MAC address — the one other devices see — isn't fixed in stone, and in fact modern OSes build in support for automatically + continuously randomizing it: https://support.microsoft.com/en-us/windows/how-to-use-rando...

The MAC address anyone can change?

Technically, yes, but realistically most random mom-and-pop places just have a random $50 router somewhere in the corner.

Would it be fair/sane/reasonable to convict such business owners if one of their customers commits a cybercrime?

What person doing illegal stuff doesn't randomize their MAC address?

MAC addresses are useless to track, you can change it, randomize it (I think even Microsoft windows has that feature built in too), or simply just throw away that wireless adapter used. It would be useful if for example these MAC addresses are tied to your identity, say when you buy a laptop/phone, you have to go in the process of adding these MAC addresses to be linked to you of some sort.