How about turning Dim Shield into a "maintained spell" like flooding? That would make 6 dims a waste against decks that don't pack much PC, and would make deflagging dims easier.
That is pretty much what Dissipation Shield/Field are, and they are way better balanced than Dim Shield in my opinion.
Would you rather have 1 dim in a deck, or 1 titanium shield? Dim is only really good in numbers, its one of the drawbacks. IT's only good if you can chain it.
It depends on how you look at it. I see titanium shield as having the drawback that you can only make use of one, but it is not reliable unless you put more than 1 in your deck. Many (not all) of the cards that increase in power as you have more, are strong cards because of that: Dim Shield, sundial, nova/supernova (more so pre-nerf), devourer/pest, black hole, silence, cloak (?).