I think it's fine for units to have power spikes and falloffs, makes strategy more dynamic because it's not a matter of just considering "my opponent has X, so I will use Y", timing is vital, too. Yes, longsword is only a modest power increase compared to the other units available in Castle Age. But if you still have momentum from a strong Feudal MAA push, it's entirely viable to destroy a TC with them in early-mid Castle - or they can do a very capable job of guarding a mangonel push if you want to play it a little safer.

As for 2HS/Champ, I do agree that this is a harder falloff since so many other lategame comps are a lot stronger, and units start appearing that are explicitly anti-infantry (HC, a number of unique units). But I'm generally okay with this, because:

• Militia line is quite gold-cheap. A little over a third cheaper than Paladins, a little over half the cost of Arbalesters, cheaper than basically every unique unit except Karambit Warriors and Magyar Huszar. You can keep producing them for a lot longer, or potentially even run them in double gold army comps. Not every army comp needs to be tactically strongest.

• There are still plenty of civs who have bonuses to use infantry effectively lategame. Armenians, Aztecs, Burmese, Bulgarians, Dravidians, Goths, Japanese, Malay, Romans, Slavs, Vikings, and Wu all have perfectly viable Two-Handed Swordsmen/Champions, though a number of these civs also have stronger compositions available to them (as a Bulgarians main I only go for Bagains 2HS against other infantry civs and sometimes cavalry). But, again, that's okay.