It feels like a compromise I can live with. I'm not wealthy enough to just subscribe to a dozen different apps, but sometimes I'll hit a cliffhanger and really, really need to know what happens next. The coins let me do that without feeling locked in for a whole month. The downside, of course, is the price per chapter adds up fast if you're not careful. I blew through ten bucks in an afternoon once on a particularly addictive revenge isekai story. Still, it's a decent middle ground between a full subscription and just dropping the story entirely.
What gets me is how they use them to test a story's popularity. If a book has a ton of 'unlock next chapter' coin purchases, the platform pushes it more. So in a weird way, buying coins for a story you like can help ensure it gets finished, which is a benefit for everyone following it.