They could be patched in...but the same can be said for many games. Why does the Sims need a new game every 5 years? Why not just release a graphics pack instead of redoing the entire game again with less features? Its pretty much the same concept.
Also another big thing is that Minecraft is a Java Game, a language which Microsoft competes against with C#. If the purchase went through, you can bet they will begin the porting process to native C++ to reap the massive performance increase. Then while they are at it, they might as well add in all the new features and by the end of the day its a massively upgraded game both performance and feature-wise.
With the better performance, they could advertise better modding support for the PC versions because now modders wont have to worry about Java crashing when it hits its virtual memory gap. Textures can get much more HD, pretty much about everyone would be able to put on those ultra-realistic shader packs without worrying if Java would Crash.