Its nothing to do with marketing........i started on mountain bikes in 1984 having ridden trackies previously. The bikes i bought saracen etc, were a massive leap from my trackie....no more buckled wheels, no more searching for downtube gears, etc etc.
But in 1990 having had several bikes, the world of kona and marin (only pick those as they had good local dealers) changed things. Nothing to do with marketing.....i rode one....wow....lighter, faster, much more responsive.....game changer.
Its not just a perception either....ive still got a couple of premium mid 80s bikes.....they are still bricks to ride in comparison to a mid range kona.
Problem wasn't the geometry then.....its my geometry now!
As for modern 29ers etc....im not taking this thread down this road, but its apples and oranges. Not designed for the same thing.....i own one,.....slow, cumbersome and used as a winter hack.....but serves a purpose.
Im sure if your crashing cliffs in utar or downhill riding they are fine....but i do hours of technical ish cross country where there benefits are more flaws imho. But i can also see the heads up, slack head angle being ideal for countyside bimbling about.....maybe in my 80s ........