Cross-class abilities were dropped because they were a bad system, there's nothing more to that one, really. If people want to try a new class, they're doing that regardless of whether they do or don't have cross-class abilities. I don't think the encouragement was that great really - and most especially when you propose for them to go beyond, say, 15 or 42, let alone if they then asked you to go to 60 or 70 for a cross-ability. There doesn't have to be a reason to level a job, beyond own self-intrigue, and trying to do so in a forced nature, e.g., cross-class is how you end up with bad systems. -- I'm not saying this is exclusively the case with every system, but cross-class in particular was.

I am pretty sure shields in the cases of CNJ and THM were just remnants of 1.x, both implementations and concepts - I'm not wholly convinced they wanted this system in, in the first place, so rather than reworking it, just steadily phase it out into non-existence across the course of the game. I mean I agree in principle, with that corresponding paragraph but the point of shields just feels... Nah.

Potential with resistances could have been great, yeah.

I don't know about Materia - If it were on the chopping block, then it would have been gone already IMO.

On the note of putting in work to support systems, I do agree on this point, and I think they should make a better effort, but I don't think all systems warrant the same ongoing support, and are systems that can or should be replaced by others, e.g., role actions, replacing cross-class, or shields... Now, granted, I do think role actions would benefit from being more complex than they are.