Since the introduction of iOS, and then Android, it's proven extremely difficult for any mobile vendor to make a real impact. BlackBerry at least did taste sweet success at one point (how things have changed since then!), but Microsoft... that's an entirely different story. While there have always been fans of the platform, Microsoft has failed to deliver any recipe that's been able to coax people en masse to adopt it, and as time goes on, the company's goals are only becoming even more difficult to achieve.
Its most recent mobile OS, Windows Phone 8, was well-received overall, but it's impact has again been minimal. In some ways, this is a little surprising. While I could barely find anyone that liked Windows Phone 7 or earlier, I know many that own WP8 phones and love them - but that doesn't change the fact that they're being squeezed into the smallest slice of the pie chart.
Given all of that, it isn't too much of a surprise to learn that Microsoft is planning to start Windows Phone 9 development with a clean slate. Will the modern UI be scrapped? It's hard to say - Microsoft itself isn't even sure at this point. There's supposedly no mock-ups, and no UI decisions. We're clueless, and Microsoft's clueless. It's safe to say that we're not likely to see WP9 for quite some time.
I am not convinced that this is what Microsoft needs, however. Admittedly, I've never been a big fan of the tile aspect of WP8, but again, I know a lot of people who prefer it. In some ways, it's actually a breath of fresh air over the similar design layouts of iOS and Android. Usage-wise, WP8 also seems pretty good.
I believe one of the biggest hits against the platform is that there are not enough devices available, nor are there enough apps. If you're an application developer settling out to design a new app, which platform are you going to focus on? Microsoft? Not likely. You're going to go where 75%+ of the market is; iOS and Android. While Microsoft's Windows Phone store is growing larger, it still can't compete with the big two, and for people who want to be assured that they can get most apps they hear about, WP8 is not the place to be.
It seems likely that Microsoft will have to begin really encouraging developers to support the platform by way of rewards. It'd cost the company money, but up to this point, everything it's done hasn't had the impact needed. Simply put, if WP8 had the same sort of catalogs as iOS and Android, I highly doubt that the platform's usage would be measured as a single digit percentage.