I think folks are also underestimating the fact that they were actively working on systems for 9.0 until... the actual day the expansion went live. While they've "cut it close" before, the delay and active iteration right up until go-live is a different order of magnitude. This probably means they had even less pre-emptive work done on 9.1 like they would for follow up patches in the past, especially with the WFH limitations.
The way the game has evolved at this point means they're basically going to have to either add some new systems or extend the existing ones. With gear trickling in at a glacial pace in many cases, and the squishes making most upgrades look pretty minimal, power has to come from somewhere. Whether that's something like the extra set of Artifact Weapon powers they added in Legion or an entirely new system, who knows, but for better or for worse, they've pivoted to most of your character's power growth for an expansion being attached to borrowed power and systems. I don't think 're-grind all the same conduits you already had at a slightly higher item level' is going to compel people to continue playing. It's already felt a little shitty getting conduits to give you back powers you've had in previous expansions.
Given the fact they had basically nothing to show, they're only just now getting 9.0.5 spun up on the PTR, and WFH gutting a lot of efficiency--although it's probably less of a factor for a game designed to be played online and across the world rather than a console single-player primary game--I'd be surprised to see it before like... July. That seems like a very long time.