Really hope not, Ubuntu is like a beacon in the Linux world, no matter if you like it or not. Microsoft purchasing Ubuntu would mean the end of Linux desktop...
I don't think it would be the end of desktop Linux but it would mean a focus away from the core principles of Linux (which Canonical was not really that good at following anyway).
Considering those collaborations with Microsoft, I've seen some reports of Microsoft wanting to change Windows so it's based on the Linux Kernel. Seems like a long shot but...
The kernel is not the problem with Windows. Arguably the NT kernel is better in design and cleaner in implementation than Linux.
The issue with Windows is 30+ years of legacy cruft and APIs that are all bundled in, mostly in user-space. Needed for backward compatibility which how Microsoft won and keeps the “enterprise” market.