"even the little 210 loader that goes on the 2210 has more lift capacity than the 419 (at different heights, of course)"
Of course, that's a rather poor comparison. One cannot use some of the specs to make a point...if you're going to compare, use all the specs.
The 419 was castigated quite a bit when it replaced the 70 loader due to "less capacity at maximum height". Of course, the 419 rises almost a foot higher then the 70 loader at around 85" give or take 3". At comparable heights and break out, the 419 has higher capacities.
If the 300 series loader is easier to remove and install, it's got to be almost automatic. The 419 is extremely easy to drop off and re-install.
All in all, the 790's capacities are lower then some of the competition. It is a really old design, but for the great majority of folks who buy it, it'll have all the power one will need for a long time.
"The 300 loader definitely has a LOT more lift capacity than the 419"
I haven't seen the specs on the 300 loader. They are not listed on the Deere web site. Any idea of the max capacity at max height (what is the max lift height?)?. How about the breakout force?