I think I'd argue against Sharepoint simply because of the investment and vendor lock-in required. You do have an extremely solid point about Google Apps at the bottom end because of this exact reason though.

I only wish the O365 platform was more stable. I had endless issues when I tried to setup a paltry 50 accounts from a different managed Exchange platform. We ended up with using the Office products using the O365 license (cost of 2013/2015 products was far too high to justify individual licenses), but switching Exchange back to the previous provider.

Also, when it comes to high availability, there isn't much that can beat the ease of SQL Server's HA solutions. I personally greatly prefer PostgreSQL over everything, but there is no doubt in my mind that MSSQL is a solid platform.