"Well designed systems" are the key words.

Code written with AI can be fixed, if well designed, they are not mutually exclusive.

Also legacy systems were absolutely not always well designed, I've picked up a fair share of systems that were absolutely not well thought out, usually due to time pressure, or any other context specific reasons.

I do believe we are creating legacy systems faster now, and it's because teams are unable to keep up with the speed of changes, not because their system is not meant to be maintainable.

My point remains tho, using AI doesn't have to mean normalizing failures.