Microsoft can probably scale Minecraft up to support infinitely large sized worlds if they want to. Minecraft lazy loads the world in chunks so you there's no real limitation (aside from machine digit sizes)
This has already occurred. This has been since Minecraft infdev (2009, now 17 years old). Later on, a limit of 30 million blocks (30000km) in each direction from the origin was added, due to floating-point precision issues and integer overflows in world generation, which get more severe past this point. Integer overflows in world generation caused the infamous "far lands" in versions prior to Beta 1.8.