SMN wasn't even very complex since Stormblood anyway because they ultimately got rid of how DoTs can be a nightmare during Heavensward and gave us so many QoLs to apply DoTs ever since.

The only thing that hindered it was clunky pet response and pet actions ghosting which surprise surprise, still exists in new 6.0 SMN. And no, this didn't add 'complexity' and people didn't like that aspect of the gameplay. It was an annoyance every SMN player wanted fixed out.

By the time of Shadowbringers, you have so much mobility that it's more convenient to play. (Though, Shadowbringers SMN is where it goes downhill for different reasons)

It wasn't a hard job to play. The people who complain either didn't bother to understand how the job works or was still stuck to thinking it was the weird mess it was during ARR/HW days.
The little amount of 'complexity' it had before is now gone, and now it's just boring.