This is a false dichotomy. Encounter difficulty will definitely raise the bar on every level. Job complexity, as long as they remain accessible, will do less so, for example. Raising only the ceiling, in a perfect world I do agree there, would leave enough wiggle room for the baseline casual difficulty not to change too much. Game didn't crater or failed spectacularly back in HW even with the most complex version of jobs in XIV history. Casuals just went by just fine. If anything, the game grew a lot from ARR which was piss easy in comparison.
The problem with encounter difficulty is that it tends to act in a very binary way: you either succeed or fail mechanics. Jobs and the battle system though? You can add enough depth that a player that wants a bit more out of the system can actually play with levers that are totally optional for clearance otherwise.
Also, I do think that players need to learn to fail again, even in casual content. There is literally no feeling of accomplishment is there is nothing at stake, because yes of course we lost friction everywhere.