I’d imagine you’d have a hard time using USB power banks to form a battery that resembles (for example) a MacBook Air battery:
Considering most power banks use 18650 cells or similar (but even if they are thinner), I can’t really see how you’d form a battery pack that fills the space effectively on most notebooks anyway.
It’s also a lot of work finding the correct cells to use (form and size wise), ordering them, if it’s in a power bank prying that apart, desoldering the old and soldering in the new cells. >= 99% of all people would purchase complete, fitting battery packs for their model of laptop.
I agree with you that the repair service can be expensive to offer, but the replacement part should still cost next to nothing. I can’t imagine a phone battery costing any more than $10 to manufacture.
What I’m concerned about is that this law is pretty useless without cheaper prices for original batteries to go with it.