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[Lymberey]

a 25k DoT will leave a lot of classes at less than half HP if they don't heal.

I believe the highest overall potency DoT without LB is 22.5K which is 5 ticks, 4.5K/tick. In 15 seconds, the probability of someone not healing is practically zero, and 15 seconds is a very long time for how fast CC gameplay is. Against competent players, it is much more manageable than burst and cc as they already know when and how to engage and disengage.

More importantly, 15 seconds is a very long time for Mana to regen which is 500/tick in total of 5 ticks. So, that 22.5K DoT is actually almost free to heal though. Any DoT lower than 15K and has 5 ticks or more are practically free. Even if you heal in the middle of it, you will get half of your mana back by the time its over, and already mitigated half of ts dmg. It is the least of your concern while being attacked anyway, unless it is the notorious triple DoTs of RDM. We dont even have to talk about other defensive cooldowns.

In case of long team fights that DoT can run through its whole duration, unless the DoT is really strong like RDM or SGE LB, it is usually just a nuisance and doesn't even matter as people can just take turn to heal as 15 seconds is long enough for a member to come back from death. This is why it is more important to CC and burst people to oblivion than applying weak DoT as it is also a huge opportunity cost for the attacker, unless that DoT has some debuffs, really strong, or you have no burst.

The only exception of this is when you can stack multiple DoTs on the same target and still has the ability to CC and burst them down, a feat that RDM can achieve solo. Long and weak DoTs are usually used as damage supplements, much like Monk during 1-6 combo but without potion interruption.
It would be a huge opportunity cost and a major threat in long team fights if there is no mana regen.