Optimal for people doing what?

I'm not disagreeing with you but are you're talking about people doing half and marathon or 5K and 10K.

Maybe am I wrong but I did my best marathon time when in shape for 10k...with lot of easy volume, little but intense Vo2 work.

I'm not much of a run guy. But wasn't the whole sweetspot thing dependent on the relative durability of the body on the bike with that volume versus the pounding on the body of running? Meaning it isn't as applicable to impact sport, like running. As it defeats the purpose of enough recovery daily to repeat.

I made my first breakthrough at Half marathon on Zone 3 (sweetspot) stuff, my second breakthrough was a few years later doing a lot of threshold.

Maybe it stimulated my abilities correctly at each time, maybe I was properly rested for each of those efforts and it didn't matter. At each time I ran 10K well for me.

I struggle with too much sweet spot work. I want to do it, but I can’t handle more than a 4-6 week block of it before switching to more polarized. I think going back and forth might be a good idea, building up extensive aerobic, time to exhaustion, with the sweet spot and then adding in some intensive stuff to drive up the ceiling. People who do a 12-16 week sweet spot base amaze me. I just get too fatigued to get on the bike for more 15-30 minute intervals multiple times per week. Especially with swimming and running. It’s like I just get constantly fatigued.

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USAT Level 1 Coach
Team Next Level
http://goteamnltri.com/

Since most people adapt to a stress in 5 to 6 weeks it makes sense to change it at that time interval. You could increase the stress or change the type.

Perhaps rotating a block of threshold or VO2max stimulating workouts would be good way to change that stress for you.