For what it's worth, that's never been my stance. My stance is that I learned this way, I'm glad I did, but I want it to be better, because I do recognize not everyone enjoys reading these sorts of things front to back like I do.
As for what could be improved:
- Reorganization of the text. Running the game first, then adventuring composition, then rules variants, then world creation.
- Addition of a practice/starter area/dungeon. Not nearly as in-depth as LMoP or DoSI
- A more thorough example of play, as previous editions featured. Footnotes or sidebars explaining judgement calls/design decisions would be a plus.
- Added philosophy on DM best practices and guidelines.
- Discussion of player styles of play, DM styles of play, and the potential benefits and hindrances therein.
But as mentioned, I'd also be very happy if 3/4/5 were instead/also/somewhat folded into the PHB.