I do agree that as an artist, you have to find a way to be okay with this. But at the same time, I think most people know that you shouldn't care, of course you shouldn't care, but the thing is that if you do care, that's not very helpful to know. Many people can't just decide to stop caring -- they can either care, or just feel guilty about caring, afraid they are bad artists because they care about their readers despite their best efforts.
I think it's okay to care. You just have to find a balance where you care, but can let go, so it's not paralysing you with you fear.
The weird thing is that you see this exact same fear in professionals; published creators who, assumedly, know what they're doing??? but the truth is, many aren't confident that they do! You see this over and over again in successful writers who admit that they feel like they're a fraud, pretending they can write, and no one has yet noticed that they can't. Feeling as though you don't know what you're doing.... doesn't actually mean you lack those skills!
Putting pen to paper and hoping for the best and getting better and better at your craft just through practicing it and trying to get as close as you can to what you wanted -- that's how most of the big webcomics made it! And most of have stumbled along the way. You have to realise that there's something in your art, something you're good at, that people like. It's not that you ACCIDENTALLY did really well but now you don't know how to do it again -- if that were true, you honestly wouldn't have gotten all those subscribers. There's something in your art that is intrinsic and unique to you, and the people who liked that thing subscribed. Those are your core audience, and you'd be hard pressed to fail them: they like your art because it's YOUR ART.
Some people who have different expectations and thought your art was going to be something else might unsubscribe later, but that's okay. It's okay to lose them, and I think you do need to find a way to be okay with some people leaving -- because you will never disappoint your core audience if you are true to yourself and make a comic you like.
As for me, I ABSOLUTELY care, and sometimes it really makes me worry. But ultimately, I have to let that go. I can't not care, but I can remember that it's okay for a few people to leave, and it doesn't mean I disappointed them, just that they weren't my core audience. There are people who love my comic for exactly what it is, and I just need to be true to that. ;u;