Pretty much the answers provided are "it's always been done that way"
Yes our answers have been that, because that's what you asked:
Provide an example of game breaking balance if they stack please.
and
My question is, does stacking make it so broken that the real hp pool never gets touched
Now, if you want to ask the question "Could Larian change things to have multiple temp HP pools and have it work?" that's totally fine, and something that could be discussed. But it *is changing the goalpost.
The answer to that question is...maybe. A lot of people already take issues with how Larian has adapted 5e rules (some of these issues are just because we want it to be a faithful 5e adapatation, sure, but others definitely have a butterfly effect of consequences that people aren't happy with), so trusting them with another change is...eh. If temp HP from armor of agathys was allowed at the same time as other HP, but armor of agathys HP was always taken away first, then ehhhh okay fine. That's probably not too broken. The only real sources of temp HP are false life, armor of agathys, rally, and dark one's blessing. And only one of those would restore itself in battle.
But, as people have mentioned in this thread, you *can combine temp HP, HP max increase from aid, and the HP shield from arcane ward. You yourself gave an example of this in your 3rd post. If this is not how the game is implemented, Larian should fix it. BUT, now we have returned to the argument that things in the game should match 5e rules. You are unable to play this wizard who gets HP from rally, aid, and arcane ward because Larian did not correctly implement 5e. :P
