I’ve thought about this too, especially after listening to the Fast Talk podcast and watching Dylan Johnson videos where he often references this Seiler’s review.
TR kind of goes against the 2, maximum 3, high intensity sessions per week since most plans (especially build ones) have three very high intensity workouts and one moderate intensity SS session. IMHO, as others have said, SS with a properly set FTP (for example with a TTE test instead of the ramp) shouldn’t be considered high intensity.
However, SS is still harder in many ways than Z2 and doesn’t provide the same adaptations in the long term, so if you have time you can swap the Sunday’s session with a Z2 workout as suggested in the weekly tips. As a matter of fact, the TR guys said on the podcast that originally they scheduled longer Z2 rides on Sunday but they saw that compliance was too low and decided to swap them for short and more intense workouts.
And if you have even more time, you could drop one intense workout and do a longer Z2 ride, basically going “polarized”. You can find a huge amount of information here Polarized Training Discussion (Fast Talk podcast & Flo Cycling podcast) and here Polarized Training Workouts & Experiences (80/20).
Personally, I tried for a couple of weeks to follow a more polarized plan, doing high intensity on Tuesdays and Thursdays, a short Z2 ride on Wednesday and longer Z2 rides at the weekend. However, I didn’t like it because during the week I’m more stressed and short on time, so the quality of the workouts and of the following recovery suffer, plus doing long endurance rides (3/4 hours for me) back to back on Saturday and Sunday was kind of boring and repetitive.
I would have preferred to do Tuesday and Saturday as intensity days but I don’t have more than 1.5 hours during the week to do long rides, so now I’m basically following TR MV plans, adding a bit of Z2 at the end of workouts and when possible (aka if I ride outside) a longer endurance ride on Sunday.
In the end it comes down to the time you can dedicate to training, the more time you have the more Z2 you will have to ride.