Hey Judge, just wanted to clarify on this and how strange your block rate difference estimates are. The logic is sound if there weren't abilities such as sentinel, divine veil, and other mitigators that completely trump the incentive to mitigate trash pulls in the first place. Like, of course you want to block more, but the difference isn't large enough to care. The logic just doesn't fit into the game's situations, but for clarification, you can't make this large of an improvement on block rate in the game at all.
For a Holy Shield 110 block rate + standard HDL 206 dex build to have a 30% block rate, the floor for that monster would have to be -12.6%. I don't think this is likely to happen on "trash monsters" but I have no idea, let's roll with it.
Floor + (Block Rate x (.2)) + (DEX x (.1)) = block rate ≤ 75%
so
30% - (110x(.2)) - (206x(.1)) = -12.6% floor
Likewise, considering you were talking about the same scenario when comparing (trash monsters, standard vs. high block rate + dex stack), let's see how much you'd need to reach 75% standard block rate on a -12.6% floor.
The increase from -12.6% to 75% is 87.6%, so you'd need around 438 block rate, or 876 dex, or some combination of the two. As you can see the numbers for this increase are just unreachable in any sense. Realistically, you could have... 196 maximum possible block rate from gear and traits. You'd still need 484 dex total.
I want to put up the math for the block power differences too, but it's pretty safe to say that to gain that 45% block rate, you'd be giving up 43~45% mitigation in block as well in the shield alone, not 14% as in your example. So basically, there's a reason we use Kite and Tower shields. Our abilities override the need for block rate. Nothing overrides the need for block power.
A more reasonable comparison would be Holy shield 110 vs. Gridanian Buckler 178. In the above example, simple switching to the gridanian buckler alone would net you a +13.6% increase in block rate (30->43.6). However, you are trading +13.6% block rate for -13.4% mitigation (Holy shield +27.6% -> Gridanian Buckler +14.2%). Numbers like these are just too small to care about for trash monsters in the game's current scenarios and ability design (divine veil).
Comparing individual things like HDL gauntlets vs. Sentinel's Gauntlets shows a clear winner (12 VIT = 6 Block Power = 1.2% mitigation vs. 5 Block Rate = 1.0% block rate), then when you meld DEX onto the sentinel's gauntlets you are just trading block rate for defense, and non-mitigation stats like HP, Enmity, and Mind, for very, very little gain.