The only, and I mean ONLY reason Blizzard has said they want to redesign or nerf prot's mastery is because they want us to always be able to benefit from more mastery. As far as they've said, it has nothing to do with how much damage we do or don't take nor whether we're able to get full Combat Table Coverage.
First, the Holy shield change doesn't affect benefiting from mastery at all. Paladins still want to cap mastery even with the current version of HS. Second, changing block mechanics is a poor way of getting Paladins to benefit from mastery because it will always be a nerf. If we block all of incoming physical damage (that is not parried or dodged) then raising requiring us to benefit more from Mastery would require us not haveing full combat table coverage. You say this isn't about full combat table coverage, but the two are inseparable when it comes to tanking (unless you are going to design mastery to do something other than just block, like every other tanking class)
If you really look at allowing Paladins to always benefit from Mastery, then it is ultimately going to do EXACTLY THE SAME THING as a Buff like Sunwell radiance. The only difference is that if they do not go the Sunwell radiance route, then all the bosses will hit the tank exactly the same. There don't need to be any other reasons why that is stupid other than the bosses should get progressively stronger as everyone moves up in tiers. Therefore, Fireland bosses should require more total avoidance/block than previous bosses for ful combat table coverage.
Changing HS isn't addressing the problem you quoted. Making higher level bosses more likely to hit the tank will.
Unglyphed Divine Protection gives you 44% damage reduction post block (1-(0.7*0.8)) whereas Holy Shield gives you 50% damage reduction.
I realize this is a thread for high level paladins in the highest tier of raiding, but I would just like to point out exactly how much this change really hurts paladin tanking at lower gear levels (pre raid). I know most people aren't concerned about that, but I think it deserves at least a *little* consideration by Blizzard.
At lower gear levels, it probably would be a better idea to macro it than trying to use it as another CD.