To your point about different sports, polarized vs sweet spot is a very different debate for other endurance sports besides cycling. Running at moderate and above volumes almost HAS to be polarized by itself- spending significant time running at SS will almost certainly lead to injuries because of the high impact nature of the sport, even if the cardiovascular system can sustain it. So if you can’t spend much time near threshold and above, you’re only option to build aerobic endurance performing that sport is to add in a lot of easier, less demanding training. On the flipside, swimmers do a ton of HIIT training and spend a LOT of time near or above threshold, even for aerobic events (which I would consider a 400 or longer). Swimming is the ultimate low-impact sport, and the only risk if from repetitive use injuries in the shoulder (which come from total volume and form moreso than intensity). It’s critical to have a high VO2 max and maintain good form for swimming, which is why their training sets are split up into short intervals with static rest.
Cycling doesn’t really have any limiters in terms of injury risk and is less taxing on the body then XC skiing or swimming, so you can do either approach with success. Measuring outcomes is also so diverse in cycling that it’s hard to quantify which produced better outcomes- meaning that even if sweet spot hypothetically produces larger FTP gains, that won’t be indicative of performance for a short track MTB or crit racers. On the flipside, I don’t think a long-course triathlete or gran fondo rider would care about peak 5 min power or anaerobic repeatability.
and I think some solid intervals are in my near future.