I'm thinking the key is volume. Z2 allows for more of it.

There is probably a crossover point where X amount of Z2 will cause greater adaptations than X amount of Z4. Z4 works faster, but how much can you do?

Even at my lower volume levels - 7-10 hours per week - If I do a hard Z4 session, say 3x15, I'm trashed for the next few days. And I can only do workouts like that for 3 weeks, then I need a break.

I can ride Z2 for however long I feel, whenever I feel like it - and never get tired or burnt out.

Who knows really what that crossover point is? Obviously for pro level riders - they have enough volume for Z2 to matter. What is that number for us duffers?