Please Reconsider Splitting Main and Off-Tank Roles Based on Job

[Terhix]

This is a bit different. Your MT will want to have enmity for as long as possible to maximize their damage output. If there's a forced swap, your MT needs to swap back as soon as possible to get counterattack procs. If the MT is not the active tank, then they will likely be expected to stand in avoidable AoEs to proc their counterattack to avoid 'griefing' their party with lower damage. There will probably be entirely new 'MT uptime' variations to every strat with different swaps once people get their initial clears and want to maximize damage (and you know how well NA/EU PF handles variations in strategy). You will see lots of PF arguments over who gets to tank and for what parts and over how it impacts their personal damage. I don't even need to see that conflict arise to feel disappointed in it.

Going through the entire Arcadion:

M1S: there are swaps for busters but you swap back immediately.
M2S: MT actively tanks the whole fight.
M3S: 1 swap for a buster by convention.
M4S: 1 swap for a buster by convention.
M5S: MT actively tanks the whole fight.
M6S: desert patch might see MT lose uptime, not an issue with 2x20s gauge.
M7S: some tether swaps during which MT still takes damage.
M8S: 1 swap for a buster by convention.
M9S: MT actively tanks the whole fight.
M10S: MT actively tanks at least one of the bros.
M11S: MT actively tanks the whole fight.
M12S: MT only loses uptime for candies in P2, not an issue with 2x20s gauge.

12 savage fights, 13 if you count the door boss as a separate thing, and in none of them this would be an issue.

Ultimates tend to be a bit different, FRU certainly has some moments where depending on strat MT isn't actively tanking, but if you plan strats with Evo tanks in mind those are things that are easy to correct. Ultimates also generally blast the entire raid with damage so frequently you can probably have 2 MTs and still get all your counters in, sans complete downtime (boss not targetable).

I can see there might be places, especially in Extremes and below, where a tank with counters eats a vuln to get an extra counter in, but this is something that already happens - not for counters but for melee uptime. In EX5 it's a common practice to drop a short CD and eat a vuln during MM1 if it saves you uptime, some guides even recommend it! I'm not going to tell you that taking avoidable damage to do more dps is a good thing to do, but if you are against counters based on the fact that it might incentivize that behavior, you should also be against tanks losing melee uptime since it also incentivizes that behavior. We already had a whole expansion where melees (tanks included) virtually never lost uptime and decided that's bad for the game.