As a person who regularly flies international with just a carry-on bag, I very much prefer to get out of the airport with my bag in 20 minutes after I leave the plane vs waiting who knows how long for it to arrive and hope that somebody didn't break it/into it.

Newer planes/retrofitted ones with larger overhead bins with space for everybody are the solution.

A lot of airlines have started doing this by "gate checking" bags.

Flying with an infant, I'm very happy I can bring a diaper bag and other essentials on board.

OTOH it would overwhelm baggage reclaim and everyone will get stuck there instead.

that's a problem for the airport. Faster turnaround for the airline though!

nope. airport gets overwhelmed, aircraft get stuck because of processing, airline costs rise, ticket prices rise.

it's a single pipeline. every single one bottleneck has to be removed.

let's start with TSA.

I've heard that the boarding process itself is rarely the limiting factor in flights. They're usually waiting on other plane-related things (refueling? Pre-flight checks? I can't recall the details).

If it were, they probably wouldn't be doing their 8-group boarding process that takes 20 minutes just to let people start boarding, because gate-time is expensive for them.