On the topic of 'greats', this sport has
no all time greats. It only has all time
most famous riders. The past & methods involved make any attempt at creating hierarchies a very difficult task.
The current so-called GOAT aka Merckx is the most famous, most iconic... but his methods preclude placing the guy on a pedestal in raw sporting terms.
Rog will have a place in cycling folklore once his career is done, with or without a TdF. The sheer number of wins to his name ensures his 'fame' in the sport (& his overall story plays a role as well, aka the ski jumper story).
This seems to be common wisdom, but I don't know what it's based on. It has however been repeated so many times that now apparently Roglic has started to believe it as well.
I'd say it's always better to be on the team of the strongest GT rider in the world, than racing against him. Except if there's no opposition, like in the Vuelta. But that's not going to be the case in the Tour.
Because when Vingegaard attacks, Rog is not allowed to jump on his wheel at Jumbo. It's as simple as that. Teammate rules say you can't drag opposition riders over to a teammate, or chase down your own teammate, or attack him if he has yellow. I mean what does Rog do if he's sitting 20 seconds (for example) behind Vinge in GC in the Tour? Launch a surprise attack? I'm sure that one would go down well on the team bus.
Leadership of another team = operational freedom. It basically ensures a Vuelta 2023 style scenario cannot be repeated.