What I found after years of low intensity riding - mostly Z2 with limited work in the other extremes.

Gains are slow, very slow. I can ride longer at the same level of effort, but not harder.
Any time spent in upper zones - like a quick sprint or a spirited short climb - ends my day.
Any time spent in the middle zones is a crapshoot on duration and recovery ability.
I could ride at say 185w+/- all day. I could easily ride at 175w for an hour. But riding at 210+/-w for an hour was very difficult.
And reached a plateau.

After just one 3 week block of intensity, VO2 and threshold intervals, my Z2 numbers increased slightly, but my tolerance for harder efforts increased significantly. I can better recover from sprints climbs, and I just did 1.5 hours at 225w & felt much better after.

Z2 has its place, but without the crazy pro levels of volume, it also has its limits.