I don't disagree, i.e. in theory you describe bike racing exactly as it is.
But in Tour history (at least since I started watching in the 90's), I cannot think of a winner who wasn't able to put bring the hammer down & dominate his rivals when it mattered, either in the TT or mountain finishes - irrespective of whether he had the best team or not. Oscar Pereiro perhaps? But that was a weird Tour for many reasons.
Indurain, Ulrich, Amrstrong* (asterisk because obvious reasons), Sastre, Contador, Schleck, Evans, Wiggins, Froome, Nibali, Thomas, Bernal & Pogacar all dominated their rivals when it mattered. Roglic must do the same this year, i.e. because the likelihood of JV pulling a tactical masterstroke or defeating a stronger rival is slim (& relies on chance to a certain degree, i.e. usually manifested by crashes eliminating rival GC contenders).
The simple math is if Roglic cannot put time on Pogacar in the TT or stay with him on the climbs, he likely won't win.