Wonder if legal problems
At least where I live, Lumia's were widely advertised with the promise of bundling 15Gb of free OneDrive storage. Taking that away might catch the eye of consumer protection agencies. No doubt there is some fine print somewhere which allows Microsoft to renege on its promise, but the local consumer ombudsman has in the past often taken a dim view of such tricks, when ordinary people are concerned.
Of course, allowing "unlimited" storage was a stupid move on Microsoft's part, I don't care if they replace that with, say, 50Tb. But 5Gb for the free account is too little in this day and age, when you can get 2Tb drives at your local supermarket for 100€. I currently have a total of 40Gb "grandfathered" from their various older free offers, when they cut that to 5Gb it is a pain.