Okay, in most cases I'd say it was impossible to get worse so long as you keep drawing...and even if you stop for a while the worst that could happen is you get out of practice and rusty; you have to remember how to do things again.
BUT I do believe it is possible to have the quality of your art freeze in place or at least improve really sloowlllyy, even if you still draw a lot.
There is a webcomic artist I've been following for years, I won't bring up said person's name out of respect for them (they're not on Tapastic), but they have been around a LONG time, it has to be pushing over 15 years now...
....and their art has barely. Changed. At all.
Okay, to be fair their art got a bit cleaner and their style more streamlined, but once it got to that point the improvement just stopped. They've been drawing backgrounds in a really lazy fashion for years and all of their angles are very, very basic. Rarely do I see experimental angles or character conversations that aren't just a load of talking heads.
In fact, in some ways, they've gotten worse because the artist has also become more fond of using copy/paste face shots for long-winded dialogue scenes and it's really distracting. Said artist must've decided early on that their style was good enough and they preferred ONLY quantity over quality, because I have never seen them make an attempt to make backgrounds more detailed or to improve how their characters are drawn by all that much.
So apparently, even if you continuously draw, it is VERY possible to fall into comfortable habits that stunts your growth. I do not mean to bad mouth this person because I do the same thing in some areas, I know there are things I need to stop and REALLY refine that I merely get by on now, I just haven't had time to, so as a result, it has only improved a LITTLE bit, if at all.
This is why I think, even with webcomics where you're trying to keep things consistent looking most of the time, you should still try to experiment when you can so you make some improvements as you go.