This isn't going to change anytime soon.

If everything is ready on the plane and there are a good number of people waiting out front why shouldn't they start boarding a little early?

It would seem crazy for everyone to sit there idle for 5-10 mins instead of doing what they can to ensure an on-time departure.

If you arrive exactly at the 35 min mark then you are going to hit this as an issue from time to time.

If getting on first is important to you then arrive for domestic flights at around T-45 and you'll often have a short wait but you won't miss the start of boarding.