People will use whatever case they want but Jens clearly stated support ONLY for the other model and if you had not read it there was some sort of argument between him and the french case makers on these forums. This doesn't stop anyone from using them though.
As for its use, why would they sell as much as accelerators? Most A1200s are still working and if they don't people get them fixed. You say "of course it is" but put forward no facts backing this claim.
How many motehrboards are actually out there (do you have a number?) and when exactly will they run out (again, do you have a date?)?
I changed my Amiga 1200 motherboard once because I did something stupid to the old one. I recapped it once after that. I also got a video chip fixed once. Still same motherboard.
I also got my A600 for 20+ years. It's been repaired at least 4 or 5 times. Still working. Why would I need a new motherboard? It's working fine. Unless I let stuff happen to it like acid leaking on it or whatever, it's a perfectly good motherboard and its failing parts can be fixed and/or replaced by a technician (we have many Amiga technicians around).
I think you are overestimating demand for this kind of product. I think it's great for building an A1200 from the ground up for someone who doesn't have one or wants an extra (you would be getting a NEW A1200!), but for the existing userbase, it'll be only for those who need to have all the things or really need this futureproof machine. Personally, at the price it will probably go for, I wouldn't be buying it. I love the C64 reloaded but I don't have one, there's no reason for me to have one.
And when this motherboard breaks, and when Jens runs out of custom chips (will he ever??), what then? This is the same "logic" you're posing. You talk about "years to come" when we have seen that Jens' products run their course and don't come back. Some times he releases an updated product in their stead, some times not. You CANNOT assure these motehrbaords would be available "for years to come" especially without inside knowledge of the market, knowledge I believe Jens of all people would clearly have.