I'm gonna call bs on this. 4e was NOT well put together. It was conceptually sound, but the design was an absolute mess and the proof of that was the mountains of errata that those products needed. The PC math needed a complete rewrite of the armor table in AV1, mandatory feat taxes to fix saves and base attacks came in PHB2. Whole powers and subclasses got rewritten (Martial Power) the skill challenge system went through two large overhauls, and the monster math was terrible until MM2 made it playable and MM3 made it good. 4e needed another year or so in development to find all the math issues that plagued those first two years, and by the time they figured it out and Essentials came, 4e had it's reputation cemented.
So I don't think 4e was well put together from the start. I think it felt rushed and the errata that was constantly invalidating what had been printed in the books and need to redo whole rule sets shows that. (If you had access and knowledge of the errata, you were playing a very different game in 2010 then if you just used the books as they were published).
Now to be fair, 5e has had some of that two: Xanathar and Tasha both changed parts of the game fundamentally, and Mordie Presents is a giant errata document turned into a sourcebook. But the underlying main game loop of 5e has been sound in ways 4e could only dreamt of being, and you don't get a second chance to make a first impression.