Looking at an M324 the spindle body could be made a little larger to accomodate a larger outboard bearing similar to M520's,
the PD-M324 outboard bearing is already ~50% bigger and stronger than the outboard bearing in PD-M520 etc. and it arguably still isn't quite strong enough for that duty (if you use the flat side and mash away). Shimano wouldn't want to make SPD pedals with very different platform heights, so an overgrown spindle cartridge assy (appealing though it is in some ways) is a bit of a non-starter.
And even if the outboard bearing did wear more often,needing more servicing,it's a whole lot better than water ingress in pedal bearings ruining cups and cones beyond repair
you misunderstand perhaps; its not usually water that damages the outboard bearings in PD-M324, it is bad adjustment, bad lube, and horrible loadings. I mentioned water ingress into PD-M324 because that is the only damage I have ever seen in inboard
bearings, and that hasn't ever been catastrophic.
FWIW I have been deliberately using a near-wrecked LH PD-M324 (which I acquired when already damaged for experimental purposes) for a couple of years now, to see what happens if you persist in trying to fight the inevitable. I took a look inside it yesterday and on the plus side, the amount of metal particles in the lube was less than when I last looked. But the outboard bearings are so badly worn that it really needs a new cup as well as a new cone. The wear patterns tell me very clearly that the damage was originally caused when the flat side was being used. The inboard bearing is , by contrast, almost perfect. The bearing was getting smoother as time went on, but I think it has plateaued now, and isn't going to get any better than 'not really good enough'. It also hasn't stopped wearing either.