I think there is some hope but I don’t really know how to do it. I think if their behavior was considered sufficiently shameful according to their ingroup then they would stop. But their ingroup specifically selects for people who think they are doing the right thing.
I have some small hope that they can be convinced by good arguments, although if that were true, surely they would’ve already been convinced by now? Perhaps they are simply not aware of the arguments for why what they’re doing is bad?
Not OP but I think Functional Threshold Power is fine. I don’t know of any literature directly comparing it to VO2max, but much of the literature on VO2max didn’t actually measure VO2max, it used proxies like “maximum gradient at which a participant can walk for 3 minutes” (called the Balke treadmill test. When meta-analyses report that VO2max strongly predicts health outcomes, what they usually* mean is “VO2max, and also various proxies for VO2max, when thrown together into a meta-analysis, strongly predict health outcomes”. So as far as I can tell from what (little) research I’ve looked at, there are a lot of metrics that work and it’s not clear which ones work better than others. And FTP seems like as good a measure as any.
For example, have a look at Table 2 in Impact of Cardiorespiratory Fitness on All-Cause and Disease-Specific Mortality: Advances Since 2009, which gives a list of studies and what measure each study used. You can see that they used a variety of fitness metrics.
*I’ve only actually looked at two meta-analyses