I'm sure you're aware that switching over to Ubuntu is not nearly so simple, so I don't need to explain hardware compatibility, backing up data, cross-platform app availability, etc. If it were really so simple, System76 and Dell's Project Sputnik would not be such big deals.
Google also competes against Yandex and Baidu, to name just two. That Yahoo uses Bing for their back-end doesn't change the fact that they're a legitimate competitor as well. It's possible Yahoo will revive their in-house search technology once their deal with Microsoft expires, too.
And speaking of counterfactuals, we can't just ignore Bing's existence because "Microsoft may tire of it". Any company would hypothetically be a monopoly if all their competitors gave up...
I also think it is worth noting that Google's only real competitor is a non-profitable Bing that microsoft literally has to bribe people to use.
Microsoft may tire of it and throw in the towel.