While the metrics used to determine the qualifications for the system seem arbitrary, I don't believe there is any honest substitution for actual experience. The people who'd "grind everything to 60" just to be part of this new system are likely going to be crap mentors. Actually I will go so far as to say like 60% of them will be crap mentors. Probably more.
The fundamental flaw here is that we, as mentors, shouldn't be teaching new players how to play the game a particular way, as in exactly like us. What we should be doing is giving them insight and rationale as to why they should do certain things or what to consider when approaching content. Having a dynamic perspective from playing multiple roles through so much content, one would hope that insight is substantial enough that we're a walking Eorzean encyclopedia of why's and why nots.
Even in this thread you've got people complaining that they shouldn't need to play a tank job to it's highest tier because you wouldn't need to know that to be a Dragoon. Well, that's a short-sighted perspective. If I can explain what considerations the Dragoon should be making on account of the tank, the possible situations they may encounter, how to manage their skills and abilities in said situations in accordance with the other roles in the party it is far more effective than saying "impulse drive until 26."
Like, as a good DPS, try to focus the target your tank is focusing since at lower levels it can be difficult to generate enmity. How do I know that? I leveled a Paladin and it sucked hard not having Shield Oath until level 40. If you're also drawing hate from another enemy the tank can't handle, the Healer's role becomes more difficult as well because they have to keep the Derpgoon who's running around being chased by wolves they aggroed alive. Understanding when to take advantage of Stoneskin in order to maximize damage output by eating an aoe is something else. How do I know what Stoneskin does? I leveled a White Mage to 60 and I know that there is clipping on Dragoon's jump frames, or that their buff falling off wastes a ton of DPS so I need to keep an eye on their health when I see them struggling with a rotation due to mechanics. How do I know the mechanics? I've run every dungeon in the game, and I've done it collectively over a thousand times allowing me to predict patterns and mitigate them. See how this all functions together?
To be a good mentor, you don't teach a player to play how you play. You teach a player how to be a good player by giving them the knowledge needed to take full advantage of their chosen job, while also providing them even more knowledge on how to be a good team mate.