$75M over two years is compensation of 75 engineers in the SV
(not even taking into account opportunity cost - this 75 folks could innovate and bring more revenue)
Some businesses are not about innovation, some businesses are about margins. Personal cloud storage is that business - people shop around and buy on cost.
I use OneDrive because it's free with Ms Office, which I need anyway, and their 'family plan' pricing is great. I decide cloud storage for like 5 accounts, backups of family photos, etc.
That is in spite of the fact that dropbox is miles better, and has extra features - they dont move the needle.
Or maybe the workload of using their own infrastructure was lower, and scales well, that the savings could go towards having 75 extra people who could innovate instead of not having them.
So either it's not $75m or it was a bad idea.