"Reboot leads to death" may be an unfair statement in the context of webcomics. LOTS of webcomics die prematurely, including the ones that never get rebooted!
That said, neverending reboot hell is definitely a thing. I think "early art looks ugly" is almost always a bad reason to reboot. Because yeah, your art is always going to get better, and your earlier art is always going to look worse than your current. That never stops.
If it's so badly drawn that you can't even tell what's going on, sure, that could stand to be fixed. And if it's ugly but still visually clear, then that's probably not a good reason to reboot? Maybe you could fix the worst of it here and there, but you don't have to throw out everything. Art improvement over time happens even in professionally published comics.
Rebooting for the writing, that's a different beast. I did this with my current comic. It wasn't because my early version was of poor quality. Regardless of quality, it was literally not a story I wanted to tell, theme-wise. The lack of planning was definitely a factor. Now,
This is why I advise to prospective rebooters, "make SURE you know what went wrong last time, so you don't end up making the same mistake again." I knew what went wrong, and I knew how to avoid it. My current version is far from perfect, but it's still a story I want to tell, with a theme that comes from the bottom of my heart. That is why no matter how dissatisfied I am with its quality, I keep going.