That's there for people who need to deal with getting strollers collapsed and ready to be stored with the luggage, and kids safely in their seats. OP's method extends into the normal boarding process, because you have to wait for other passengers to get on and be willing to move their seats.

Also, the early boarding is for whatever people interpret it being for. Most airlines are intentionally vague about justifications for early boarding. "If you need extra time or assistance to board...", and lots of people take advantage. I know a particularly "chutzpah" guy (who is young, has no disability, nor children) who always boards during this period because "nobody's stopped me!"

It also applies to small children, in my experience. But if you're flying with a toddler it's a lot easier to convince the gate agent to help you sort it out before boarding begins.

Early boarding does nothing to help with different assigned seats. It actually does the opposite because no one else who could consent to a seat swap is on the plane

I've had the stewardess notice my kid in an adult only seat (turns out only one side of that plane has airmasks that fit kids or something like that) and move us, then tell whoever we displaced to take our old seats.

If I was a flight attendant, I’d use this reasoning regularly even if it wasn’t true. Hard to disprove on the spot. And these kinds of things can even vary from order to order of the same aircraft between (and within) airlines.

The idea is to get noticed early, if you have a seat assignment issue, or get a row if it's a free-for-all seating airline.

Again, it's entirely different if it's a toddler/small child vs a teenager; teenagers are probably happy to be away from the parents for awhile.