Nadella hasn’t overseen creation of a single new service or product. He has merely cut/focused.
Ballmer should get far more credit for current stock market success.
His maniacal animosity toward open source and Linux in particular made no sense and caused people to move away from Microsoft's uber-proprietary stance.
He can get credit for some of the things that got started on his watch, but had he remained CEO for the past five years, I believe Microsoft would not be prospering as it is, and would be both a technical and business laughingstock, despite its remaining grip on the dwindling desktop market.