I think one of the only 10+ year old comics I still read is Gunnerkrigg Court. 
I've been a huge fan of but still dropped all sorts of things I read back in the day, like Questionable Content, Darths & Droids, Order of the Stick, and even the thing that got me into webcomics in the first place, Megatokyo. I think some of the most successful webcomic creators at keeping my attention have been the ones who finish a series and start something new and different, like David Willis who went through various different comics with the same characters but reimagined in new scenarios, with the most recent being Shortpacked! and Dumbing of Age, or John Allison, whose big hit was Scary Go Round, which he eventually finished but did spin-offs in the form of Giant Days and Bad Machinery in which the previous main characters were now the older supporting cast, and the kid characters were now older and going to university and high school, so it was recognisable, but fresh.
I think that approach is pretty smart because a really long webcomic, like one that doesn't end after around 5-7 years tops, tends not to be just a single story, but tends to be more like a sitcom or a series of episodic stories or arcs, where uh oh, what wacky thing is happening to the characters now!? And there's nearly always some kind of expiration date on that, where you run out of new things to do with the characters and you can either get repetitive, change the characters in ways the audience may not like or relate to any more, or introduce new characters the audience might not care about so much.
A bad thing that can happen to a creator is discovering that their core audience will stay invested and they'll make money from their IP practically no matter what they do so long as they just keep generating content until they bore most of the audience. There's a little voice in the back of every creator's head that says "okay, you can't be too self-indulgent. Don't waste the audience's time! Edit your overview/script! Kill your darlings! Keep it tight!" and for some people, it's a problem because it gives them so much self-doubt they can't even put pen to paper (or stylus to tablet), but if it gets drowned out by too much praise or demand for content from the hardcore fans, that's also not good. When a creator stops curating which arcs or scenes should go in their comic based on what takes us towards the conclusion in the most entertaining way possible and just puts them all in, things just really start to drag when a comic updates just a couple of pages a week.
