Is it a cashgrab or a good crossover? I'd say it's both.
I think crossover side of things does make at least some sense from perspective of lore of original Gridman and Dynazenon, but ultimately still feels a bit forced, and possibly unnecessary. Most of crossover elements were "just because why not?", and did not bring much to the plot, if anything at all. Besides having 3 (or 4 depending on how you count) "good" robots in the final fight.
But even aside from that it felt... Subpar to what I remembered from both original series, that did tackle some serious topic, practically existential crises even. This one... It had main theme of "are they going steady?", which did not even make sense, because no one uses that phrase before people start going out together. But maybe that was just peculiarity of the subs.
There was nothing besides this, though. There was a point early in the movie, when it started feeling, that it has a chance to become psychological, but they did not go that route and abandoned any hints of that all-together, even though watching Yuuta go insane would have been interesting. Probably. It would have been more interesting than robots fighting lackluster monsters and requiring numbers just because... Plot, I guess? Previous shows has monsters that had unique powers, had rules, that robots needed to figure out. Here it was just "no, they are just too strong, we need help!".
Gridman's route was also wasted. Not to spoil things, but he was changing, and that change could have been either very interesting, and could have worked very well with psychological elements in general and with Yuuta seemingly going insane in particular. Or it could have been as insane as Gurren Lagann was with robots throwing galaxies at each other. But it was neither. Again it was there for the sake of being there.
In the end I felt like I wasted 2 hours on this. If it was 1 hour, maybe I could accept it, but not like this. The crab looked good, though.
