But a tactics module of complex play rules to match 4e would be great.. . . y'know, what I'd really like is 2e characters (with kits) in 4e combat. That might be my ideal D&D.
Backgrounds, in 4e & 5e, both, are like the simpler Kits in 2e, and 4e Themes were comparable to the more substantial 2e Kits. 5e is entirely open to custom backgrounds, too. So all it would take is turning on all the optional 'tactical' rules, expanding some classes' tactical options, tweaking monster math to allow combats to last long enough for tactics (other than Alpha Strike & focus fire) to have an impact, and creating Backgrounds to mimic some of the more interesting 2e Kits.
And what do you mean by "wide audiences of players who aren't into character optimization"?
Character optimization is a system master activity, it's great fun once you know the system inside-out, a sort of depth-of-play issue (it can be good or bad, a system that can handle it gains depth of play from optimization, a less robust system loses it as most apparent options are discovered to be non-viable). Not everyone strives for, let alone achieves that level of system mastery. 5e doesn't go out of it's way to reward system mastery like 3.x intentionally did, nor out of its way to restrain it with mechanical balance the way 4e did, rather it sabotages it at the basic level by giving them no solid/definable system to master - 5e is fluid at the whim of the DM, you'll have to master him, not the system.
To counter any narrowing of audience (in playstyle), they simplified it to try to maintain or even grow audience.
But, y'know, an ever-growing audience sure doesn't sound 'narrowed' to me.

Whether it's because of the game itself, or riding on the coat-tails of the rise in boardgame popularity, IDK. I'll take it.
5e is a reactionary kneejerk that has sent the edition pendulum crashing back toward rpg extreme from the power gamey 4e.
Yes. But it seems like there is a big resistance to any sort of change in that direction, which FEELS like this:
"yay! We won the edition wars!!! Now don't give an inch back to the other side - they screwed us in 4e! Make those power gamers suffer for it!"
OK, so there is some truth to this, I'm sure. 5e is trying to be conciliatory in the post-edition-war era, and consolation can often be mistake for capitulation. But, really, 4e was hardly the powergamers' dream that 3.x was, the game had already started pulling back from the peak of intentional rewards for system mastery, and, 5e is far less robustly balanced than 4e, so you can powergame like crazy with more significant results - the DM just has unlimited licence to pull the rug out from under you when you do. True powergaming in 5e is gaming the DM, not the system.
Let the rpgers have theirs, let the power gamers/tactician/miniature enthusiasts have ours.
First of all, we're all RPGers, whether we like telling cooperative stories, portraying/exploring/developing a character (capabilities and/or personality), or plumbing the depths of play possible in a complex game. It's RP /and/ G, incomplete without both.
For me what sucks also is that 5e got so much right. They really did a good job imo of fixing the caster-non caster imbalances.
Relative to 3.5, when those imbalances were at their absolute greatest (Tier 1 vs Tier 5), perhaps. Compared to the heavily-restricted, fragile magic-user of the early game, maybe not so much. If we recall how closely-balanced martial classes were with 'caster' classes of the corresponding role before Essentials...
The multi-class caster rules are fantastic - but so simple it makes me wonder how i never stumbled across that concept as a house rule.
I was so pleased with the return of 3.x/PF's elegant MCing system. I do seem to recall a number of such ideas being discussed on line back then, and I even used something of the sort in my 3.0 campaign.
I am a student/officinado of games, and 5e just does not have a finished product smoothness to it. It's like there was a team of designers and each guy went his own way with different sections of rules. Perhaps there wasn't a strong leader/final editor that unified it all before it was complete, perhaps it was right-brain thinking rather than left brain.
It seems like Mike Mearls was all but working alone, and was certainly the lead, so I doubt there was disunity of the sort you speculate about. Rather, my guess would be that what you're seeing is a consequence of 5e as a (The!) 'Big Tent' edition, trying to be inclusive towards fans of each and every prior edition...