This type of clear been possible for a few expansions, but it becomes progressively easier with each successive expansion.
If you want a more concrete example, take a look at this
run from the Dawntrail Media Tour. It might be subjectively stressful for the tank doing that particular solo, because of how many mechanics they get hit by. But it's also clearly not all that difficult. They easily heal themselves up through two vuln stacks. Difficult would mean clean runs only, and having limited healing tools that you had to plan out judiciously. What we have currently flies in the face of skill expression. People used to get genuinely impressed at WAR solos. Now nobody bats an eyelid because of how common it is.
Speaking generally, nobody wants to sit around and spectate a tank clumsily and slowly solo a boss while getting hit by every mechanic and with multiple vuln stacks up. That clear took 10 minutes from 50% to zero, and there's nothing you could have done as a party to terminate that pull and reset. That's a genuinely terrible situation to be in, and I'm sure you'll have more than a few newer players lose interest if that turns out to be the standard dungeon experience for them.
I don't think there's a problem with sustain actions existing on tank (assuming that they're designed appropriately), but the balance around them has become progressively more skewed over time, secondary to the defensive powercreep that we've seen across all supports. It just needs to be reined in a bit.