Been mountin biking for 35 years, I have a kickstand on my bike. What argument against them is there?
Respectfully, I have to ask what you consider mountain biking? I'm not trying to be mean but I have a suspicion that you're not doing real mountain biking and maybe just riding on gravel roads or tame dirt paths. Do me a favor and look up the Whole Enchilada in Moab, UT and let me know if that's even close to what you're doing while mountain biking. I think it'll make more sense.
They're extra weight that can get caught on things and break your frame or you. I've never seen a serious mountain biker riding with a kickstand.
* the geometries of bikes have changed dramatically. Bikes are longer, with slacker headtubes, longer reaches and steeper seattubes. 29" wheels are finally being crowned as "the" standard, with 27.5 and 26" wheels being relegated to speciality bikes.
*The style of riding has shifted from XC, where distance and ups/downs are to be expected, to more Enduro-style, where you go up to go down. The downs are what your bike is oriented to do. The downs are also getting more 'extreme' in the sense that you're seeing lots more flow/jump trails or simply highly technical sections.
Kickstands are not used (I haven't seen one affixed to a modern MTB in a decade or more) because they're heavy, they're a moving part, and most of the routes I ride they'd be an annoyance. They might be useful for you, but you're not going to find any manufacturer touting them as standard equipment (or even compatible) today.