Fair enough. But, again, I'd look at the specific rather than the general.
Does anyone actually believe a Ring of Warmth is overpowering or broken? It's a pretty minor magic item that I don't think any DM in history has ever worried about dropping into the game. But, technically, it does allow an "always on" concentration effect - a 3rd level Protection from Energy spell. Any "plussed" item works this way as well.
So, the notion that allowing multiple concentration spells isn't necessarily a problem. The problem is, how do you predict the interactions between multiple concentration spells? Is the world going to come to an end if Silent Image is allowed to stack? Probably not.
OTOH, it's probably too much of a PITA to comb through all the spells and figure this out.