Don't get me wrong, in principle it is good, but the cross-class system itself at the time of implementation was heavily criticized and disliked which indicated they didn't like it, and for a good reason, it wasn't one of those systems they just aimlessly went headfirst in and removed for no reason, there was genuine feedback against that system. Edit: I would say given their development or lack thereof in Heavensward shows how they themselves felt about it, even before it was replaced by role abilities.
Like, it's well and good saying in theory it gives people an opportunity to try new classes and pads out the content, etc., - But you need to consider that more people than not were not treating it as that means - It was more like -- "Wait, I need B4B on my MNK? You're telling me I need to now level LNC???" - With a sigh ensuing.
Cross-class was only really reworked into the role-system which we have now, which as I have said isn't a bad system, but really could have a little more to it. - Role actions were their idea of 'improving' cross-class, which between the two I would take role abilities.
This is also all well and good saying, but IMO for cross-class to be what it could've been, would have inevitably needed to be reworked in the first place, and honestly if I had to say so there's really external systems that already fight-back against what cross-class would even have a chance of being, e.g., the game from head to toe is very rigid, and always has been -- Probably always will be too, which doesn't entirely favor a system like cross-class, especially when you might want to look at it in the perspective of "Oh, it could've been an enabler of sub-jobs, or something that 'blurs that line between Tank/Healer/DPS".
A lot of the earlier systems, especially those present in ARR were absolutely just about throwing out ideas and systems and seeing what sticks - That's just how bad the initial product was in the first place. Many of them inherited from the ARR days, and some others also tried to derive from FFXI. Your last point would reign true for absolutely every system in the game - But again, the problem in the case of cross-class specifically is that people disliked it from the premise - All you'd be doing here is substituting deletion of a system with trying to ram it down peoples' throats (Neither are good). They already had a good swing and a miss on that with Diadem and the Specialist system for crafters. - What do you do in the event you keep developing it, and keep going, and going to just find out that people still don't like it? Keep wasting resources on it?
Looking at some of your other posts, I don't exactly disagree either in principle, but cross-class just ain't one of those systems.