I agree with their model in general, volume matters a lot and you’ll plateau fairly quick without it. You don’t need to increase to infinity, bc there are diminishing returns to volume just as there are with intensity. There is def a big genetic component that the model misses, by definition. These regression models are typically finding the best fit line through the data, so produce the average effect of some explanatory variable on the outcome variable, by definition. Genetics are captured by the ‘error term’. But to say that ftp increases with volume would hold empirically, for sure. If you’re lucky, you can high a high fitness on sub 10hours per week, but you’re not going ‘pro’ without 15-20+hours per week plus the right genetics. I’m sure most people couldn’t get 5w/kg even with 15-20hours per week bc genetics matter so much, tho they could have really great gains relative to their baseline. If you don’t have the genetics for it tho, you probably won’t have a high w/kg without the volume.
In my experience, I never have sustainably had volume beyond 8-9hours per week, but I have been fortunate to just have a disposition to be good at sports.
When I started cycling in my early 20s (fit from other non-endurance sports), I went from about 4w/kg to 5w/kg in 1-2 years at 75kg, then plateaued on ftp-type gains ever since, but continued to increase repeatability for the next 2 years. All at 8-9 hours per week structured, 2-3 ‘workout days’ while racing (which counted replaced workout days).
Took a few years off any structure and probably did 3-4 hours per week of not-necessarily-cycling aerobic exercise to be fit.
I got back into cycling 4 years ago and have averaged about 8 hours per week (as usual) the whole time. I started at around 4.5 w/kg- from previous fitness- and hover just below 5w/kg. I pretty much got my general aerobic fitness back in 3-4months. I have tailored some of my training blocks to hit power PRs in certain time durations. Ex, max 20min effort (5.4w/kg), 5min (6.4w/kg), 3min, 1min etc. I have produced big numbers which ftp estimators would suggest my ftp is way higher than I could actually hold. I’m just not a classic endurance sport body type and haven’t don’t the volume to develop like that.
All that to say, you might be lucky like me and be really strong compared to ‘normal’ riders on relatively low volume, but still won’t hang with the ‘big boys’ unless you get that volume in. And quite frankly, for a non elite athlete to train 8-9 hours per week regularly, that is quite something and consider a lot of volume to any normal person or even typical cyclist.
Think of the math too, 6 to 7 percent change is a lot bigger than 9 to 10. 6 is quite low volume too. You would prob want at least 4 days per week at 1.5hr+ as well as one of those at 2-3hr to ‘build the engine’. You’re not going to be super strong doing a 60min ride everyday, regardless of intensity. You need bigger stimulus to the body, in my opinion