Um, that example isn't that good considering Noivern was at least in the reach of Zapdos at least to that extent that it could damage it. So it's not appropriate to use that as the example.Noibat cannot possibly get experience from Zapdos, since its bound to get curbstomped by a legendary because its not strong enough..... oh... wait.... it did get development, since it battled it.
Secondly, that did look somewhat DEM. Noivern, who hardly managed to give a decent performance afterwards, somehow managed to deal significant damage to a Legendary Bird, doing better than Talonflame and Hawlucha? That performance of Noivern against Zapdos seems to be an outlier, characterized by DEM factor.
A Pokemon can develop by losing, sure. But only if the loss is a decent one. If the loss is a decent loss, then they can get development. By common sense and logic, Rowlet and Litten are nowhere within the reach that they could give a half-decent fight against any UB. So hardly any chance of development there.
If Rowlet and Litten are able to give a half-decent fight against the UB's, it would seem to be a complete cheap DEM (even more than Noivern vs Zapdos). You really want Rowlet and Litten to get development through cheap DEM? At least I don't. It would be better if Rowlet/Litten get development during fighting Totems/Kahunas, because there they have a chance of putting up a good fight.
Rowlet and Litten need to fight stronger opponents, sure. But the opponents shouldn't be that much strong like the UB's where they can possibly stand no chance, like the UB's. If, for the sake of plot/development, Rowlet/Litten manage to hold its own against UB's, it would seem like complete cheap DEM.
I'd take Greninja coming back over that anyday, because that would be a far more realistic way of fighting UB's
Explained above. A Pokemon can get development from loosing if only the loss is a decent loss, not a curbstomp. Rowlet and Litten are currently within no range to avoid any curbstomp against any UB's.A common misconception is that a Pokemon only gets development by winning, but also if its loses it gets development.
You said Lycanroc won't be effected, so that's it. No problem in Greninja returning then.What don't you understand about Greninja returning taking away development from the REGIONAL team. Permanent or not, that doesn't matter. If it appears, it takes away development, it doesn't matter how you look at it (Because that is shown in Johto, with the starters, Phanpy and Heracross. All of them didn't get the needed development because the old team was around. Noctowl is less of an issue, since it was a fully evolved mon and thus already needed less development) If Litten and Rowlet never take on stronger opponents, how do you imagine they grow stronger, well... they won't since training doesn't give you BATTLE experience. The only one of his regional mons that isn't gonna suffer from a Greninja return is Lycanroc, since it already got development and needs less development since its already fully evolved.
Rowlet/Litten aren't within the range currently to avoid a curbstomp against any UB, so that battle experience is redundant, as I have repeatedly explained above. It needs to be a decent loss to get development. And even to get a decent loss against UB's it would take something PIS or cheap DEM.
Battles against Totems/Kahunas are the ideal ones for Rowlet/Litten to get development.
So, the Ultra Guardian arc is a good time for Greninja to return, where Rowlet/Litten can't get any possible development, plus it would give Ash and co. a far more realistic chance of standing up to multiple UB's.