As opposed to a boring, passionless, static, stagnant 24th century setting?
Yes, TNG/DS9/VOY certainly stuck to that vision. Who can forget Picard's monotone, milquetoast delivery of "I say now, there are four lights, Gul Madred, care for a spot of tea?" or Sisko's polite and measured way of dealing with his crew?
You couldn't have the characters that you have in DSC in this story if you set it in the 24th century. Now, you can argue the merits of the DSC characters all you want...but NOBODY would buy these characters in Picard and Janeway's antiseptic and morally superior time frame.
There should have been more interpersonal conflict on TNG and VOY. But there's also something good about a society that aspires to be better than it once was. But aspiring to greatness is not dark and gritty, so...
So they set it around Kirk's time when human beings were still recognizable as such.
They set it around Kirk's time for fan service.
So again...you can argue the merits of those choices, and you can argue all about whether it is the right or wrong approach, but you can't say that the setting was only done for marketing purposes (although that certainly had something to do with it...the TOS era is far more recognized and marketable). It was essential to the type of story they wanted to tell and the type of characters they wanted to tell the story through.
I wouldn't say it was only done for marketing purposes. There's the fan service aspect. And even if the writers felt humans in the 24th century were just too darn perfect for this edgy, dark, gritty tale, why would it be so hard to believe a "morally superior and antiseptic" human society could change after years of warfare? Would that be too hard to believe?
You can believe the show is the best Star Trek in history and that's fine. But 'true' Trek didn't die out in 1987. Some of us like the 24th century just fine, and would love to see what happens in the 25th, and so on.
The human adventure continues...until it's too boring, then we'll just go back to Kirk and Spock. Klingons!