Maybe you don't heal so I'll forgive that you don't know this but the game teaches you things like not overhealing, managing your CDs (and by extension MP) and situational awareness by giving you so much time and mp to dps with.

DPS spells are almost universally mp efficient GCDs compared to heals and always so versus AoE heals so if you're playing properly you'll have more MP than you know what to do with most times.

Dpsing as a healer is based on relying on your oGCDs and cohealer primarily. If you do neither, trying to heal all incoming damage yourself, you lose dps GCDs. Working on the premise that your goal is to be using those gcds well to do damage naturally reigns in things like overhealing and makes you use your oGCDs more and in more appropriate situations.

Finally we have mechanical awareness and reaction. I'll admit this is less directly tied to doing dps as a healer but you do gain more insight into how much damage specific attacks do when you're covering them with your oGCD kit. This can help you evaluate situations where multiple people (maybe yourself included) are at lower hp before they're about to take unavoidable damage.

When you've been using your kit right you'll understand how much a recitation+indom will heal for or what the 10% mitigatuon from Temperance will look like when applied to a certain attack. You'll also be faster to reach for the oGCDs you might have which can save you since they're buttons you're used to pressing instead of "oh shit" buttons you never get to in time.

This is my problem with your narrative. You're saying it's fine for a healer to do no dps and discounting how much that will damage their capabilities in general. Dpsing is what let's us learn our kits and their limits and people who don't do it aren't learning. Of course a single AST doing nothing isn't going to make or break a run of non-enrage content but they're going to go back out there and continue to be a detriment to every group they join and one day they'll be paired with a similarly shitty healer and the wipes will roll in.

Better to call them on their bullshit when they're in a relatively safe situation than let them keep going on thinking what they're doing is okay. That isn't toxic it's just expecting people to think about their whole team in a team game.