Yes, I've been a WHM main since 2014 and won't be switching anytime soon. I've been there through the really bad times with WHM and good times as well. I just enjoy the class. I enjoy the support role in a group. I enjoy how it plays. The only part I don't like is the gear look but that's why we have glamour.
I noticed a couple of things in your post that are factors for you that aren't factors for me. I don't care about playing optimally. That's not saying I don't try to play well. I just don't do content that requires that sort of razor's edge min-maxing. I have everything to 80 for the amaro but I do have some go-to dps classes. If the optimal rotation isn't fun, I just don't do it. I find a comfortable medium so I'm still a credit to groups and we're still downing big pulls and bosses quickly.
In the past I was a BRD secondary and now I'm a DNC secondary, so having that backup DPS is another way I can stay WHM since I can pitch in with DPS for friends to just get a different experience here and there. DNC is probably the only class that has come close to rivaling WHM for my main spot. It's just fun to play. I like that it has less strict rotation and more of a priority system on what to push when. But my heart still belongs to WHM.
The other factor I noticed was that you're worried about diluting the pool by playing the same class as everyone else. Who cares? They certainly don't. If you like it, play it.
If you want to do content where optimal matters, you can always reserve a secondary class that you're really good at for that and main something else. I usually do Hunts and FATEs on DPS because it's more fun.
But the bottom line is, it's a game. You should enjoy it. If you aren't enjoying it, then stop doing whatever is making you unhappy. Many moons ago, I used to push myself to log in when I didn't want to. I used to spend long nights for little reward. I enjoyed the people I was doing it with, but not the content itself. So I only do what I enjoy here. I still love the challenge of things like Savage content, but you know what? I get my fix on that when groups go to hell and I pull out a win by being a great healer. The cover charge to be able to do that content is more than I want to pay these days, so I don't.
There's also nothing wrong with bopping from job to job as you enjoy different things about them. Do what makes you happy.