Why do you over-exagerate things? You said it yourself:
ONE dungeon. Is not “becoming too common”.
The behaviour of some is inexcusable. You are 100% correct there.
But trashing the whole community because of 1 bad apple is not fair either. Look, if you were a DD, he would have called you “DD noob”. If you were a veteran expert he would have called you noob. If you were tanking as well…
Its just that sometimes there are some bad apples. That is. Has nothing to do with you healing, or being new or veteran.
And there is nothing you can do about it except shrug it off. Leave the dungeon. Find a new dungeon. DONE.