Overall, the general upgrade paths include upgrades each other era, which is totally enough. The exceptions are:
1. Both classes of cavalry have a renaissance gap of 2 eras. i believe that's intentional and is trade off for Horsemen and Knight effectiveness.
2. Anti-cavalry units have the same Renaissance gap, which is not fine as R&F fixes it. I believe that's continuation of the previous point as Pike and Shot will take its part in making both cavalry classes obsolete at that era.
3. Scouts have long jump, but that's ok, since they aren't combat unit. Ranger, in turn is a combat unit, so additional upgrade for it is very logical.
4. Melee Naval has 2-era gap between Galley and Caravel, but it's actually 1 tech between them, so I wouldn't consider it to be the gap.
Overall, with R&F fixes, the structure of upgrades looks great. All military units have upgrades in 1-2 eras or less, so they have time to last, but not enough time to obsolete completely. The only exception is 2 Cavalry classes, for which mid-game ineffectiveness is a planned part of balance.
A multiple upgrade option would fix the issue we now have with obsolete units
1. Multiple upgrade option only makes sense if those upgrades are from different unit classes. Which, in turn, makes an issue with promotions on class change. This was a big deal in Civ5, where upgrades with class change were quite common, especially for UU.
2. I don't fully understand what the issue with obsolete units is. The current upgrade path requires more strategy to play with, but that's good thing not an issue. Yes, AI upgrades units incosistently, but even if it's a problem (and I don't see why it should be), additional unit upgrades will not solve it and could even increase.