Players generally avoid tanking roles because they require responsibility and a thorough knowledge of the fights. Other roles allow you to focus only on the mechanics that directly affect you. So while dps often have a gcd-by-gcd understanding of their own attacks in a fight, and while healers know the timings of the incoming damage, you're the one who needs to understand when and where everything happens in the script so that you can set it up and put it all together. There are more mechanically complex roles, but none with the same knowledge requirement.

You couldn't ask for a higher impact role. But you have to have the motivation to do it. You can't convince someone who isn't already interested in the commitment. Even newer players who aspire to be tank mains sometimes opt to playing dps roles into new fights, in the hopes of bypassing the learning curve and watching how it's done first (this is a really bad idea, as the only way to learn how to tank a fight is by doing it yourself.)

If you think that tank cooldowns don't make a difference, then you are either tanking content that is pitched at a lower difficulty setting, or you're doing it well after gear and echo softens the checks. It means that it's time to tackle something harder, or at least find a group that can do it earlier in the content cycle.

Not really sure what you mean by surviving tankbusters in A12S without any cooldowns. The entire fight is doable with essentially zero Grit uptime, but a cooldown rotation is certainly required. Punishing Heat probably hits for upwards of 40k unmitigated, and Holy Scourge is 4-5 hits of 30k or so (you might just about survive the former with just Grit + Virus, but you probably want at least 30% DR for safety). Unless you're talking about A12N, of course, which was designed to be steamrolled on release day by the average player with i240 gear.

As an aside, but I would like to see physical/magical defense cooldowns (i.e. Sheltron/DM) moved off of job specific actions and on to the reworked role-based cross-class set for tanks to pick and choose on a fight-by-fight basis. I'm still not sold on them, because I feel that short duration cooldowns tend to oversimplify cooldown rotations. But if they're here to stay, I'd rather not see them be the deciding factor in raid comps (the same is true for slashing/piercing debuffs, but I suspect that's the direction that we're going in anyways).