I remember listening to the video and he was talking about "Eldritch Tactics" where instead of a cantrip as a bonus action an Eldritch Knight would have this ability that said "Cast a cantrip and make an attack". Immediately I saw a problem that if he devoted more time to his answer I'm sure he would have seen. By 11th level, as currently designed, you would be changing three attacks and a cantrip for one attack and a cantrip, which is a massive reduction. This would be fixed... unless part of the point is to weaken the Eldritch Knights offense. Unlikely, but I spotted the problem as soon as he said it, and I'm not a Lead Game Designer with a massively successful game. Clearly just from playing the game I couldn't, you know, understand some of the ways it works and the systems involved.
I think perhaps the problem here is taking an example that is simply intended to give you an idea of the sort of mechanic he's talking about and then extrapolating it out of context. Yes, that would be an obvious issue. So, yes, the full implementation would be designed in such a way that this wouldn't actually be a problem. He's not suggesting you could just drop 'Eldritch Tactics' into the current 5e ruleset.
I think the entire problem with trying to discuss this is that the change is substantial enough that it would require a full reworking of the whole system. It can't be done as a quick patch or a quick UA as an alternate set of rules. It's why Mearls said that it's the kind of thing they would only do in the context of a new edition. Which he has said is very far away.
In other words, there's no point in trying to extrapolate his 'Eldritch Tactics' across levels or to the system as a whole. Because Eldritch Tactics would only work in context of a new ruleset. Which we don't have. And hasn't been designed. He's just giving an example of the type of thing you might see in that hypothetical ruleset. I suspect he's thought about it comprehensively enough that-- in a one on one conversation-- you'd be able to ask how he'd handle such things. But you won't get that in an interview where he's limited to giving a fairly quick response. That's a full dinner conversation (which would bore the heck out of most of the audience I suspect)/
As I noted in another thread, however, I can see where he's coming from. Just DM'd a game this past weekend where bonus actions and spellcasting caused a bit of confusion. Not a big deal. Not game breaking. But it happens often enough for me to think it'd be nice if they ironed it out some day.
Cheers,
AD