This is a bit long. For one, if we are talking about why a subclass is more of a tank than the others, we will talk about the WHOLE subclass. Melee abilities, supers, stats, and everything else. You make valid points about the bubble being stationary, but not only can you keep a whole fire team alive if they play off of the bubble, you can keep [u]yourself[/u] alive too! You act like the buffs for the bubble don't count towards yourself. Now let's talk about armor of light, which is the better comparison to radiant skin. It gives about the same damage resistance as radiant skin, I'd argue it gives more but maybe it's a little less. If radiant skin is better it surely isn't as far off as you think it is. The only problem with this is that it is stationary, but it allows for survival if you stay in the bubble and hop in and out to reset the buff if you run out of the over shield or if it is very depleted (pro bubble tip right there). Now the contrast is blessing of light, this gives a weaker shield but allows for mobility. Again, you can reset the shield if you hop in and out allowing for more coverage for you. By playing off the bubble, defenders can control zones and tank plenty of damage on their own. And you bring up the fist of havoc, but we aren't even discussing it! It is less of a tank than the defender, however it shows more offensive potential as well. I'll still argue that it is more of a tank than Sunsingers because of the perks unstoppable giving max armor and juggernaut for its obvious reasons. Plus juggernaut would be out of super, what do Sunsingers offer besides flame shield? While we're at it, let's talk Sunbreakers. They have the least potential of being tanks out of the three, yet they still can be! Thanks to the sunspot build, I can unleash literal hell and be granted an overshield as I walk from sunspot to sunspot (the overshield resets at each sunspot too). Not only would I be tanking a lot of damage, I'd be dealing just as much back. The only drawback is the amount of hammers/super duration but that's extended with the sunspot build. Sunsingers have radiant skin and flame shield. That's about it. And you talk about the defenders being more supportive, look at the rest of the subclass! Song of flame and Fireborn are very useful supportive perks. Looking at warlock as a whole, they're not really tanky in general. Maybe voidwalkers with life steal but there's really nothing else. Titans are tanks, they have higher armor for a reason. Sunsingers are certainly the next closest thing as it offers the play style. This doesn't mean the whole subclass is a tank.

I can see that being a valuable reason why it's in game, but the shield thing needed some looking at. I think y'all will be fine