I personally don't want the game to be rewritten, for a variety of reasons. Particularly, modding things written in C# (the language Microsoft would most definitely choose) is not as fun or even marginally as easy to accomplish, as you can see for other games like Terraria. Many of the capabilities we have now would simply be lost. There's no doubt that a completely different team would be put on the project, leaving it to be an uninspired recreation without any of the original feel. There would be zero compatibility with everything that exists currently, other than perhaps third-party tools which might still be able to edit the world saves, assuming they don't rewrite that as well. And Microsoft taking such an active role in the development would likely mean other changes, such as to the EULA.
As it stands, Mojang is already slowly stripping out aspects which tie the game back to its roots. It's lost some of its charm along the way as a result. 1.8 is a particularly bad version in that respect, notably with mobs, where even after three patches they still float around on seemingly abnormal gravity, and can barely move in water. But it's not actually the gravity, it's somewhere else, perhaps in motion smoothing code for entity movement updates, because I never found the culprit before when I tried to fix it myself. I stopped bothering when the community made it clear that 1.8 wasn't where they wanted to go.
Modders' resistance to go to 1.8, where a large majority of the game is actually still the same, is a pretty good indication of the reaction we'd see upon an entire rewrite.