I have no problem with that. But you are taking liberties with what the rules mean by "wielding a shield." They very explicitly spell out that it's not enough just to hold it in your hand -- it's actually worn on the arm. That's a huge difference, and probably the thing that sets off my cheese alert.
The rules treat the shield as armor for all intents and purposes because it is a worn item, not something that is casually picked up and carried. It takes actual game time to put one on or take it off. It cannot normally be disarmed because it is strapped very securely to your person.
An item that is secured to you in this fashion is considerably more unwieldy than what the rules refer to as an improvised weapon. If you pick up a shield and start swinging it around, yes, you're using it as a weapon. But you didn't take the time to strap it on, so you're not wearing it as a shield (per the "donning and doffing" rules), so you don't get the +2 AC.
Conversely, under "improvised weapons," it says you must wield an improvised weapon "in one or two hands," not strapped to your arm, so RAW you cannot use a worn shield as a weapon unless you have an ability like Shield Mastery that tells you you can.
I completely agree that a shield is armour. I completely disagree that I'm taking liberties in what I mean by "wielding". If a shield is strapped on, you can still bash people with it. Actually, a well fitting shield is extremely natural to use - effectively, you backhand the target with a lump of (traditionally) steel-rimmed wood. Is it something people should be able to use as well as a weapon without training? Of course not. Is it something that should be possible to use as a clumsy, improvised weapon? Yes, of course it should be. As to whether you're holding it in one or two hands, it's occupying one hand... and if you want to nitpick, pretty much every shield design involves a hand grip, because otherwise there is no way to stop the shield rotating on the arm.
As to the AC bonus... it's hard to attack someone who shoves a shield in your face. Trust me on this (yes, larp experience isn't real fighting experience, but it's not entirely irrelevant. Most of the larps I know don't actually allow you to attack with shields, mind, because it's too easy to injure your target). 5e doesn't have facing rules, so there's no rules framework, right now, to hang a loss of the shield AC bonus onto.
It was said there were facing rules in the DMG, along with more complex combat rules. I expect that those would make a good place to mechanically fit a loss-of-shield-AC-bonus rule - maybe there's already support for such a thing in them. My main objection to it in baseline 5e is the extra accounting that it costs the game, especially since 5e has done such a good job of streamlining that accounting away without losing much mechanical depth. But that doesn't mean playing some games with much more complex options wouldn't be fun, and that's definitely where adding a consideration for losing AC bonuses would provide some interesting tactical effect.
But yes, feel free to tweet. I'd be interested to see the result. I won't, however, consider it valid if the question doesn't bring up the Shield Mastery feat. Mearls' Twitter responses don't have the reputation of being particularly well researched.
When I checked, I saw that Mearls hasn't answered many (possibly any) in the last month or so, short of the Reddit AmA he did. Maybe he's a bit burned out on them after that

Chris Perkins has pointed rules queries to Jeremy Crawford. We'll see if I get an answer.
As to validity... well, it would be an official answer. Whether we use the official rules, however, should always be our own business. Right?