Some interesting points. It seems unlikely that Tera helps defense more than it enables offense in the most power crept generation to date. As far as flexibility in the teambuilder goes, though, that would be a good a point. There are only so many matchups you can prepare a team for. Tera lets you cast a wider net, so to speak.
On the other hand, we had 8 gens where we didn't have that ability and it was mostly fine. Gen 9 is the most powercrept gen to date, though, so I could understand the argument that it might be different. But if that's the case, what do we do in generation 10 and so on? The trend is powercreep. Are all future gens just going to be unplayable without Tera to help the teambuilder? I don't think that would be the case.
And going back to gen 9, which is the discussion that is really relevant, I also don't think that would be unplayable. Without Tera, it's easier to predict opposing threats because they can't do things like just decide to counter their counters by changing type... or not. And you have to guess not just what, but when.
As far as the dichotomy between gen 8 and 9 OU, I personally find it frustrating. Heck, I find the dichotomy between some decisions in the same gen to be frustrating. Maybe what happened in gen 8 is why Smogon shifted so much this time. But I can't help but feel certain things become more about popularity contests than consistent standards sometimes. And that bugs me. I can see why folks find Tera to be very fun, but again, fun is subjective. It isn't a reason to decide to ban something or not.