Its Blizzard fault,
you cannot mail heal and do everything else in game comfortably like even questing,
so no.
And biggest no, you cannot just swap spec and become good DPS, its not happening.
So no. Ive tried, trust.
I leveled as Disc before it was fine, even as Holy it was fine, just boring as hell.
You can be a main healer and play the game, I did for 10+ years as a Priest main.
However…I was in a Guild, so if that’s your thing, you may need to find support buddies that’ll help you if you want to access group content, cus pugging just isn’t it.
Even then if I needed, switching to Shadow didn’t end my world, although I sucked at it so I’d go Disc if it got real bad. 
You don’t need to be good at DPS if you main a healer, just be a good healer and you’re golden.
Good luck!
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I always play healer, even if you can’t kill quite as fast as a dps in mostly trivial content, nothing can kill you. Oh no, the mean dwarves are hitting me! casts riptide