I really hate to see Xmarks shutdown. Been a happy user.
For not going the charging user route, could it be that they took some rounds of funding and expanded their company structure and infrastructure greatly to the point that it couldn't be sustained without a huge success? They expanded into the search and other stuff besides bookmark sync'ing. Those are the ones that don't pan out and dragging down the whole company.
Instead of shutting down everything, it would be better to keep the bookmark sync part and dump the rest, and start charging users. Not sure whether the investors would go for the low return route.