Office 365
But that's a great value for your money, if you can leverage the family plan and want the online space for backups, at least :)
LibreOffice is free...
Nice how you have ignored the 1 TB of online storage for backups.
But we digress...
I would guess the most talented hackers on the planet are targeting Microsoft's cloud services. What is in the EULA about what they are doing with your backups? Are they mining the data you upload? Are the selling it to advertisers? Cloud backups are fine if you encrypt them before they leave your network with software they aren't providing the code for.
From my experience, there often isn't much difference in quality between FLOSS and proprietary software.
If you use an Apple or Android device, you use FLOSS. Even high end appliances such as F5 load balancers tend to run on Linux or FreeBSDs. F5s even hook into the Linux firewall to perform a lot of their functions. This forum is running on free software. The main blog is WordPress based, which is free software which is probably running on HTTPD or NGINX, which are free software.
No, you don't always get what you pay for with software.