The roots are in genetics.
No amount of training (specific or not) will make some people have 5 wt/kg… Wont happen, but it is a nice thought. This is like saying if I lift weights long enough I will be “Arnold” strong…clearly there are genetic limits for maximum capability. This is not to indicate you can not keep working to find your maximum potential, but be realistic.
Also high Vo2 is (beyond) a great indicator like Chad says, but he sells it a bit short IMO. Yes not all winners have the highest VO2, but ALL race winners have very high VO2. So again genetics.
For the sake of it, I blew a 61 on and oxygen mask (in low fitness 4 years ago), around that time I would estimate I had a FTP around 270-280 at 180-185 pounds. I was weight training heavily and just riding mountain bikes for cardiovascular fitness (did not use power). Now I am 162-165 pounds while training, 155-160 in race season, still do weight training (because there is more to life then wt/kg and looking like Froome; heaven help it if that lad had to pick a small child up), and have upped my FTP to 330. I have been training for 4-8 hours a week for 2 years , with some weekly breaks here and there, nothing longer then 2 weeks. In the fall I did and outdoor real world one hour test and completed 319 watts for 60 minutes (REAL FTP). I did complete a traditional 20 minute test on the trainer recently in December and held on for 358 watts (barely), which using the 5% rule would say I am closer to 340. I split the difference and feel it is a good spot to train around (oh and my Garmin spit out a vo2 max of 66 maybe close?). So my Vo2 maybe increased 10% at the most. All aside this puts me at 4.4 - 4.7 range. From and equation stand point I need to either increase power to 364 watts FTP OR drop weight below or at 68 kg (150 lbs) at 6’1". Both a big ask… I carry 170 naturally very easily, and look “thin” at 160, ten more pounds…woof, 150 seems like a stretch for my composition, not worth the ROI may be my analysis.
On the power side that is a 10% jump essentially, and at this point I can not see it based upon many limits, maybe ekk up to 345 this year on a outdoor test. So best case in my eyes is 4.8, and enjoying life and my kids…
End of the story quit chasing specific numbers, find balance, and be thankful if you had good parents passing down the genetics, then put in some work and “run what you brung”!