True, but you seem to be focused on a Muchkinesque corner case.
I'm focused on what I think is a problem that needs to be fixed. That is, after all, the point of the playtest, to point out potential problems. People often find plenty of "corner cases" to min-max their characters.
We can debate how much of a "corner case" this issue might be, but it's not really relevant. Whether the stacking issue affects 1% of players or 100%, it's still a potential problem that should be looked into.
And I think you are making a kneejerk reaction to reasonable levels, and panicking about monsters being ineffective when full equipped in currently theoretical equipment when we have not seen Giants, Dragons, Demons or Devils yet.
Have I not provided enough thoughtful analysis (even if you disagree with it) to demonstrate that my reaction isn't "kneejerk" and that I'm not "panicking?" You seem to be implying that my position is irrational and purely based on emotion. It's not.
As for the stats of higher level monsters that we haven't seen yet, I have two things to say about that. First, the bounded accuracy design philosophy that they're using promises that high level monsters will not have vastly greater stats than lower level monsters. Assuming that they mean what they've said about that, we can expect the difference in attack bonuses and AC between low and high level monsters to be much smaller than in the past.
Second, without a general stacking rule, there is no limit on player AC. Players can effectively have an infinite AC, the only limit on it being the number of items they are able to obtain. Even in 3e with its infamous christmas tree effect and several different keywords, there was still an effective limit on how high players can go. Right now, 5e is even more liberal in allowing high ACs than 3.x was, and that's saying alot. In 5e, there is no limit to how high it can go. None.
And it's not just AC, either. Nothing is stopping people from having any number of stat-boosting ioun stones orbiting their head, either, allowing people to have effectively infinite ability scores, even though natural ability scores cap at 20.
I'm just not seeing the multiplicative synergy you seem to be implying.
"Multiplicative" is not a word I used. What I said was that two items that stack can have a bigger impact than either item would by itself.
Currently that seems to be the plan since the Spellguard Shield does not have one, and shields are not listed in the +1 armor section.
Not all magic armor has + bonuses either, elven chain being one example. I don't know whether they intend to have +X shields or not. I hope they don't. I was only mentioning it as a possibility.
And they close off design space if they institute a default rule non-stacking magic item bonuses policy. In fact two of the items (Defender and Dusty Rose Prism) would be rendered worthless right off the bat.
If they deliberately design an item to stack with other AC boosting items, they can put that in the item's rules. There can always be exceptions to general rules. Right now, the general rule is that AC bonuses all stack and an item must specifically state that it doesn't stack, as with the ring of protection. I just want to reverse that, and have the general rule be that they don't stack unless the item specifically says otherwise. No "design space" would be closed off by doing that.