As someone who publishes my own Chrome extension with a similarly sized active user base, I am impressed by the number of reviews this has on the Chrome Web Store. There are 126 starred reviews and 40 text reviews while the extension has an active install base under 4k users. That equates to roughly 3% of users for star reviews and 1% for text reviews. That is much higher than anything else I have seen. The author must have hit on a niche that is more engaged than usual. That is a good sign for future monetization options if that is part of the plan.
Having something "solved" does not mean there is no space left for different approaches or sheer competition.
And seeing people putting effort into something and delivering software is at least one indicator that the original problem is not solved in the eyes of some...
Indeed, despite the fact that Chrome/Chromium has built in bookmark and password sync, there's plenty of room for competition. Maybe some people don't like the fact that you have to have a Google account for the built in syncing, maybe others just don't feel they get what they want out of existing solutions.
My bookmarks constitute my entire knowledge-graph, and one of the scariest things to me on the horizon is Google Chrome's new "Material Design" based, card-based, Bookmarks. <shudder>
xmarks need an account, and I've had an unfortunate data corruption experience with them.
Here's my stand-by solution for the day Google Chrome switches off the classic (i.e. current) Bookmarks Manager... and yes, they're better together: