The only really long running webcomic that I still read is freefall, and that has existed in some form or another since 1994 or earlier when it started as a newspaper comic for a very brief period.
It then rebooted into a webcomic in 1996 and has been updating three times a week ever since. It has, as of today, 3,421 pages. Mark Stanley has rarely missed an update.
freefall exemplifies the exact things you are concerned about. It is long and it is slow and you cannot see the end in sight.
But at the same time it has story arcs to it, story arcs which actually do conclude. It took 20 years, but the overarching Gardner in the Dark
plot line actually resolved and a massive shift happened in the comic because of it.
If you binge read it then you can actually see many plot points foreshadowed years, decades even, before they become apparent. So he actually does have a story planned out well in advance and is mind mindbogglingly patient about getting to it.
It helps that it is not about video games. Webcomics providing commentary on video games are more like a blog in comic form than a comic on the web.
Also, if you ever start reading a webcomic and realize you don't like it, you can always just not read it anymore.