I'm inclined to agree with you, but it will be interesting to see what happens in your new world order. In reality, most players use their castles (almost) purely as treb factories - I certainly do. What with all the changes though, I wonder if this will still be viable...? Could a good
mêlée general outfight a treb-happy general with better use of better foot soldiers...? I will be keen to find out.
I don't wish to stray too far off topic by investigating the ins and outs of it because I don't think it's that straightforward - I'm not convinced it is the treb itself that is the problem. On small maps, ie few castles, a couple of trebs aren't really a problem - if anything they become a good focal point for the to and fro of battle. On larger maps however, 15 trebs running wild become a lot harder to police (ie they take over the game). In their defence, trebs are one of the few things that can keep an early gunpowder rush in check if, like me, you get bored of gunpowder rush because of all it misses out on.
Sorry, I have gone off topic... but you started it! Joking aside, I do have one sensible suggestion worth thinking about. I have just compared the rosters for monastery and castle. With monastery, there are so many other recruitment or tech choices that could be justifiably considered worthwhile compared to a treb - in the castle, perhaps not so much. A handful of mercs, a sprinkling of gunpowder, and some niche units that are frankly not worth it cf a treb. (Granted, elite foot knight is a beast, but a somewhat limited beast readily available everywhere).
(I appreciate these two buildings serve different purposes, but) maybe if some other powerful options were added into the castle roster, trebs would become less of an automatic choice and more of a calculated risk...?