Simplest answer: You can never own Office 365. You’ll be paying rent to MSFT as long as you use the product.
I also understand that 365 uses the cloud a lot. Another security risk.
Check out the 365 work-alikes Libre Office or Open Office. The software is free. Download and try a copy. Throw it out if you don’t like it.
#19 Microsoft is also testing RENTING Windows 10 too.
If that happens I will switch to Linux.