Yeah but, how long do mainframes last? Think of the COBOL systems used in government. No reason to update them, they worked forever; their job is discrete and they performed it well enough where intense updating wasn't a requirement.

You also need to ask: How much do mainframes cost? They were engineered for backwards compatibility and reliability, with built in redundancy you don't find in consumer hardware.

AI models are changing every other day. I have to rebuild llama.cpp from source regularly. We are no where close to a personal "AI mainframe."