Had It/Lost It

My Spider-Man in 2010.

There seems to be this strange thing that happens in comics where an artist will actually appear to get worse as time goes by. There are artists whose art I've loved in the past, but when I see their most recent stuff, the only thing I can think is, "What happened?"

I'm talking purely about the art, not the story or content. The line work has lost something, and it's not just that it's not fresh to my eyes anymore, it's that it seems qualitatively worse. Where I was once pulled in, I am now pushed away. With some artists it may be deadlines that are forcing them to work faster than they would like, but a lot of the time it really just looks like the artist can't do it anymore.

My Spider-Man (and Gwen Stacey) in 2012.

It almost feels like watching an athlete lose their skills, but athletes' bodies just give out, and they are not physically able to do the things that they once could. With a comic artist, I wouldn't think that wasn't the case. Drawing isn't particularly physical demanding, and it would seem one could do it for a long time.

My Spider-Man in 2015.

So in my head, it would appear that it is something of a choice on the part of the artist. Do they look at their work and think it is much better than what they were producing before? Or is it just that they can't be bothered to put in the effort anymore? Or is there some undefinable "it" that they have lost?

Like in Trainspotting:

Sick Boy: It's certainly a phenomenon in all walks of life. 
Mark "Rent-boy" Renton: What do you mean? 
Sick Boy: Well, at one time, you've got it, and then you lose it, and it's gone forever. All walks of life: George Best, for example. Had it, lost it. Or David Bowie, or Lou Reed... 
Mark "Rent-boy" Renton: Some of his solo stuff's not bad. 
Sick Boy: No, it's not bad, but it's not great either. And in your heart you kind of know that although it sounds all right, it's actually just shite. 

Does this also hold true for comics?