To me, the biggest problem with job simplification is the inevitable reduction in gameplay variety. During the large content drought at the end of ShB, I was able to continue playing actively without the game getting stale by trying out different jobs for easier content. I already knew the fights inside and out, but having meaningfully different stuff to worry about for each job spiced them up just enough to keep them from being boring.

Unfortunately, I don't think I would be able to do the same in Endwalker. As a current MNK main, right now the only job that changes up content in a meaningful way for me is BLM.

Given the fact that the majority of the community only plays casual content, making some jobs more complex would be a benefit to its longevity. Having something to improve upon is a good reason to come back to content you would be bored of otherwise. At the same time, DPS requirements are very low to non-existent, so messing up a complex rotation or dying does not affect your ability to clear in any way.