"We've saved about $2 billion a year by building our own cloud" claims Bank of America CEO. outline.com/nDEM3d
Number like this are impossible to prove or disprove. How are you measuring staff compensation, opportunity cost, hardware depreciation, amortization of real estate, and most notably: public cloud cost?
Are you assuming you'd pay a few billion a year to AWS or Azure or "hahahaha a major US consumer-facing bank on GCP yeah right" at retail price? That you'd just cut them a check for that much and not get massive discounting?
Were this @wellsfargo I'd immediately disbelieve the entire story because trusting them is a sucker's bet; with @BankofAmerica I'm content to leave it as "show your work."
You can save money by building your own environment in some cases, but it inherently slows your feature velocity.
One of the best stories of "build our own data centers" was @Dropbox. Not because they saved money, mind you, but because otherwise they'd have launched their terrible new desktop app far sooner.
Instead @dropbox gave up some feature velocity and gave us another year or two with Good Dropbox instead of the now-current Crap Dropbox. It's hard to say whether they saved money on it, though.
They say they saved tens of millions a year, but then again they couldn't very well say "it was a boondoggle and we regret every minute of it" either.
In conclusion, make your own choices; what other folks are doing matters to them, but shouldn't necessarily be what you care about. (Sorry, Gartner.)
Oh, and hey. “Turning off 130k servers” does indeed save money. 
