Overall, I think even if 3e were more like B/X and less like the deep and heavy system it became, then I think the current edition of the game would probably still look very similar. I think the problems would be different, but I think the foundational issues like martial vs caster balance would still be present.
As a side note, I really think 3E was a product of its time for D&D, both limited by RPG design at the time and the state of the existing community. The designers were still the ones that had been there for some time and knew 2E inside and out, and there were hundreds of playtesters - the majority of whom were apparently old hands at the game. I remember several articles from the designers stating they wanted to make more drastic changes than they did, but the were reeled back by the playtester's reactions (I distinctly remember the monk being affected by this). We got the 3E that we got partly because of what was known for RPG design at the time, and what the community wanted to retain as tradition and what they were willing to allowed to be "fixed".
See, I think the fundamental problem is that they looked at the AD&D 1e and 2e PHB, AD&D 1e DMG, the Options series, and then all the market research they did and noticed... that nobody was using remotely similar rules sets. The longer each table had played, the more bespoke systems that table had invented for itself and so the more incompatible that table became for everyone else. AD&D in 1998 truly was a hobby much,
much more than it was a single game, let alone a product you could sell.
That becomes a problem with 3e because... what rules do you convert to 3e? You've got all these bespoke systems. But they're so arcane that basically nobody uses them as written, if they use them at all. The problem is that without a consistent system, people don't design consistent solutions and don't develop a sense of how to ad hoc things at the table. So they either use things that don't really work right, or they just... don't use anything. So... what rules do you really
need? They had absolutely no way to really tell.
The 3e design team clearly arrived at the result of, "we want everything to use a consistent system across players and DM for PCs and NPCs." And at the time that seemed like an amazing idea... because it had never been done for D&D! And the d20 check vs a DC was robust enough to do basically everything. So... what if we just converted basically every rule?
In the end, that didn't work well. It turns out that you
can't build a consistent deterministic world engine with a set of rules where none of them are longer than a paragraph. When you try to do it, then you just spend too much of the game reading the rulebook instead of playing the game. But I think
someone had to try it at some point. There had to be something between Phoenix Command and AD&D, and how do you tell where you should be?
I think the biggest mistakes of 3e were:
- A la carte multiclassing
- Prestige classes
- Spellcaster scaling and power level
- NPCs using PC rules
- Item creation and shops
- The "full attack" rule and descending attack bonuses
And I think only 2, 4, 5, and 6 have been addressed by 5e. And the solution for 4 has been "vanilla bag of HP" which isn't great.
Where 3E suffered was it wasn't tested above 10th level properly, and given a pass because "the math worked".
I don't think the 3e team were particularly concerned about play above 10th level. I don't think any edition of the game really has been, with the possible exception of 4e, although I think that game just tried to make level 30 be roughly as powerful as 10th level PCs. Which I think is correct because the spell effects of high level spells in the other editions are basically magic items that nobody else can get.
The longer I play the game, the more it feels like B/X had the best class design. Even if that's focused on dungeon crawl survival horror and not high fantasy heroic adventure. I'm just ready for something else.