Having worked with browsers for a very long time, I'm just going to point out that making a modern browser is exceptionally difficult. There are LOTS of hidden costs and gotchas ranging from CVE protections, to back-end service supports, to even just getting anyone to work on it at all.
That's one of the big reasons a lot of folk just glommed onto WebKit. Someone else is footing that particular bill.
It's not impossible, but it ain't easy.
Everything looks small from a distance.