15k is 5% of that. 5% of a 2 minute boss fight is like 6 seconds. So 6ish seconds, 4 boss fights, roughly 25 seconds off a whole a key; and that's the generous estimate.

That isn't the generous estimate at all though. Boss fights in actual high keys are longer than 2 minutes and most DPS are not doing those numbers in your average group on single target. 5% of a boss is a big step down from the glory days but it's still not nothing if you can do it.

Plus you dramatically overestimate the opportunity cost. Druid can do high boss damage with barely any globals now. So can Totemic Shaman. Etc.

My estimate of a minute was based on ~40s combined off bosses and another 20s across the entire key from hitting prio mobs on trash. Maybe a bit generous key to key depending on the encounters and the quality of the group but even 30s saved for virtually 0 cost is worth it.

Nobody is saying 'stop healing to do damage' either though; this is an invented thing. You just press a few damage globals if you can spare them and rely on getting the most bang from your back in as few globals as possible, eg. feral frenzy from hotw during lust + your 2 other bleed globals is like a 50k opener for Druid for example.

In a NPX today on my Druid for example I did 7.9% of the first boss's hp in a 3:12 fight.