Honestly I don’t think being weak to skirmishers per se is the issue with Rathas, their problem is that their strengths don’t justify such a weakness.
For example, Conqs and Mangudais are weak to skirms as well, but being great to micro, having good mobility and high attack more than makes up for that because all of these three are things is what you want on a CA unit.
What Rathas offer you boils down to being a tankier (but slower and clunkier.
Have you tried microing these things?
I have, it’s horrible 11) CA with the ability to switch to a better version of a generic cavalier.
The switching capability is cool, but as hussite wagons have shown us, just because a mechanic is cool it does not necessarily mean it is useful.
As for the tankiness, Bengalis have access to the tankiest of elephant archers, so why would you invest into castles (except for Paiks), when you can just go into archery ranges? “But you see Rathas on fortress!” Yes, because you start with a castle so it is not that hard to go with them.
So, do I think Rathas need some kind of change?
Yes, but I would work towards something like increasing their ranged attack or just giving Bengalis thumb ring, mostly for the accuracy (of course this would mean changing Paiks a little, like making the attack speed increase less on ranged Rathas and Elephant archers, while keeping it the same on melee Rathas, Battle elephants and Armoured elephants)
That said, I do agree that Bengalis are weak to trash in general, but then again that’s an issue with the tech tree, not the Ratha itself.