Certain Titans can be nigh-unsinkable in Vanilla but only is played right.
Titans are are just glorified ability casters in Vanilla. Hardly powerhouses people make them out to be.
Anyone who knows how to properly fight a Titan would just use tons of bombers and certain capital ship abilities to laugh at the thing.
Rushing for a titan at least in competitive play is a waste of resources if not properly secured to do it.
Sure, titans can be countered easily by spamming capital ships and bombers only. But that doesn't mean it's a fun or sensible design choice. It's certainly true that building nothing but carrier cruisers and capitals is a pretty hard counter to titans. But in most mods and regular Sins, titans absolutely eat everything else for breakfast. Typically if you're throwing normal cruisers and frigates at them, the only thing you're accomplishing is leveling them up.
So yes, titans are easily countered by bombers. But I don't think it's fun that my strategy once a titan enters play has to be "Don't build 90% of the ships you have access to." Which makes me return to my original point - titans should be powerful. But they shouldn't be unstoppable and they should be very costly.
I always liked the way Sins of a Galactic Empire handled theirs. They were at the end of the research tree, which is what has been suggested, and things like the SSD were incredibly powerful, but it also required an exorbitant amount of resources and fleet supply. And despite its firepower, it could still be killed relatively quickly if it was unescorted.