I think it is more to do with the weak implementation of shields.
A, B, and C won't matter if swapping from shield to another weapon wasn't such a low hit to defense in every edition of D&D. The increase in offense has to be factored in the decrease of defense.
An extra attack, even if situational or weak, is often better than +1 to defenses vs a massive d20 roll.
Well, I mean ... saying that they've made DPR too good ... is the flip side of saying that shields aren't good enough.
It's like an arms race, but while DPR is getting more and more (including finesse weapons that you can add dex damage to, feats for two-handed weapons, etc.), for the most part ... shields haven't kept pace. If anything, they've been nerfed, given that AC was more valuable in TSR-era editions, and magic shield are capped at +3 in 5e.