Interesting viewpoint. Personally I've always found plenty to do on Guardian in 140s content between interrupts, aoe interrupts, corruption removals, building/executing FMs on key targets, Shield-walling fellows in danger, and building/executing Break Ranks; so the idea of maintaining a hot spot on top of that does not really appeal to me.
I think it has to do with the fact that I found the old agro system much more engaging. All of the things you described rarely happen at the same time, but threat recruitment is a constant process, which is very much simplified in the current mechanics.
So not only do you have to apply and maintain 4 bleeds on a target (that has enough health not to bleed out/die instantly from Rupture damage), but you must hope the tank has positioned it within your personal hotspot or that your hotspot is ready to be replaced. That's quite a lot of work in order to meaningfully differentiate it from other class' 1-button dps increase capstone. And a bit risky considering the game's tendency to "throw" hot spots in unexpected places.
I don't know. Of all the tanks, I've always appreciated the Guardian most as the one that allows me to act and react to fight mechanics the fastest. Hot spots are sort of the antithesis of that, thematically more appropriate for planners like Captains and Lore-masters, and I suppose Brawler tanks. Adding the single most high maintenance hot spot in the game to both remaining lines is a big shift I'm just not in favor of as a new core mechanic when it could just as easily be a new/renewed FoS feature.
Given the change in starting parry response skills, it would take no more than 5 seconds to inflict 4 bleeds, and even faster with an active Battle-fury (unlocks response skills for 20 seconds).
In a sense, I agree with you, the fastest possible use of this debuff would require additional actions. But I don't think that there will be any difficulties in case of its consistent use in rotation when dealing damage to a boss or a tough mob. As I see it, the validity of hot spot mechanics (at least for red guardian) will largely depend on the effectiveness of the debuff on incoming damage and its interaction with yellow line talents.