As for Do As Infinity, their later music was at least a good attempt to mimic Dai's style. It is really very similar in structure and sound. I actually think "NEED YOUR LOVE" is one of their best albums, albeit probably their most commercial sounding.

As far as solo artists: Well, like everyone else said, promotion costs a lot of money, and if the sales aren't there, you can't continue with promotion. misono & Van Tomiko, as much as I love them (VT especially), didn't have particularly strong music on their own. There is something to be said for those who are great team players, but not great leaders on their own. Making the group-to-solo transition is very difficult for everyone. Look at all of the NSync members, Spice Girls, and Backstreet Boys who were at the top of the world in groups, but failed miserably on their own.