I think you underestimate the power of +4 and an eco bonus which lasts the whole game. It is true that by default Celts will not beeline towards cavaliers even if they get +4, but the whole point is that the +4 armor will play a very huge role in transitioning to imperial age, especially when you have a solid eco bonus. We see Celts going for knights in castle age 7/10 times, it is not the bulk of their army but it's an unit which serves a purpose, to use mobility and raid, or to support the strong Celt siege pushes (looking at you Hoang).Well, I suppose your case would be the exception than the norm. Why would anyone go default cavalier without BL, than the composition Celts shine at, which is halb + siege ?
It wouldn't be a cavalry oriented civ though, but just getting plate barding. They would still lack bloodlines, and has no bonus for cavalry.
At best, this would be giving them the option of going (a weak) Hussar in late game
In my eyes, I view Celts as a strong civ vs mesos as a counter to eagles. I also see Celts fairing very very good vs archer civs (except britons or chinese), because the play for celts is MAA ranged units in feudal age, try to do damage, open xbow/skirms in early castle and then boom into Siege Ram - Woads. This last tech transition is the awkward imp transition we have seen Celts struggle all the time, and that is why players instead go aggressive and add kts + fwd siege in castle age to push (looking at you Hoang). If you give Celts +4 armor, they will be even better at this play, crazy strong pressure followed by +4 Cavaliers to raid. The only current counter play to this is a faster imp with bracer for archer civs, because that counters both mangonels and knights. But with +4, Celts should just be winning in such positions. (I will mention going halbs to counter celt Cavaliers also is awkward because Celt Infantry is just better so you are playing into their hands)
Lastly, I do think Celts struggle vs cavalry civs like Franks, Slavs, Lithuanians, Berbers or Huns, lack of mobility being #1 reason, but if your civ can simply outspam knights, celts need to do something whacky like pike siege push, leaving their base fully vulnerable to raid on open maps. Also a Cav Archer transition is something Celts struggle with big time (Another reason why giving +4 would make it somewhat easier for Celts to deal with these, not saying it would work, but it gives them a fighting chance)