"D&D is objectively bad but millions of people play it because they are too dumb or indoctrinated to know better" is not even worth debating. It is an objectively silly argument. As is the claim that a work of art can be "objectively" bad.
The statistic you may be thinking of is WotCs advertising copy that some tens of millions of people have "been exposed" to D&D, like, y'know, watched a live stream at least once.
But even if it's a million or 10 or 100 mil, popularity does not prove, nor even imply quality.
1.3 billion people smoke, for instance - that doesn't make it healthy.
D&D is a bad game, but it's a beloved game in spite of that. Same can be said of Monopoly, and it's been the number one boardgame for
generations. Being bad is not a bad sign for D&D's future success or popularity, at all.
Do you realize what you are arguing?
That DMs don't have the authority to highlight one class during one session and another class on another session. You are arguing that DMs can't decide (or at least incorporate their idea of) the encounter times and places. You are arguing that DMs can't, well... DM.
I didn't intend to present an argument, just a consequence of the statement I was responding to. If there's no prescribed 'day length' and short/long rest ratio for the DM to use as an average to structure his campaign to, then the classes presented in 5e, which are balanced around such an assumption, simply aren't balanced. In order to have complete freedom to run any day length, you'd need to have classes that don't care about that factor. (Or, like 13th age, a recovery method that doesn't care about day length.)
Now the whole encouters/short-rest/day balancing act does assume an average not an exact day, the idea being that, if you have a particularly long day, the daily-heavy classes will peter out and become less effective overall, if you have a short day, they'll dominate. Similarly, if you go a whole day with no short rest, the short-rest heavy classes like Warlock, BM, & Monk will languish, while if you have a short rest after every encounter, the Warlock will absolutely dominate. That's exactly the 'authority' you're talking about, and it doesn't repudiate day-length as a balancing factor, it depends upon it.
But, TBH, it has issues. Like, WotC's own surveys indicate that single-encounter days are very common, and days greater than 1-3 encounters almost unusual, in actual play. (Of course, they also reveal play gravitating towards the lowest levels, Tier 1, at which most classes have little staying power.) Speaking of staying power, the idea of long day balancing short days makes sense mathematically, and if you, like, compare caster slots to expected rounds of combat in a very long day - they'll end up using cantrips a lot more, which will bring down their DPR... realistically, though, in the very long days it'd take to 'average out' single- and 1-3 encounter days, the whole party is going to just run out of HD, & hp...