Once, maybe; Windows' business model has changed, they are only charging for Windows 10 OEM now and have stopped trying to publish a new paid version every few years. Their business model is recurring payments now like Office365 and OneDrive, but also ads inside of applications and I presume selling your usage data to third parties.

Non OEM copies of Windows are like $200+ (unless you buy the fake/scam keys from amazon recommended windows sellers)

You can still run the 10 upgrade assistant and get it for free.

Were OS upgrades ever a significant profit center for Microsoft? Prior to the free Windows 10 upgrade, most users I know of would only get on a new version of Windows when it came with a new computer.

Those upgrades werent free, they were hidden in the cost of the machine

I never said they were-- those are new licenses, and accounted for by Microsoft as OEM sales.

I'm asking about Windows upgrade SKUs-- the kind that you'd buy in a box someplace like Best Buy or Circuit City or Office Max back in the day, completely separate from a computer. Were those ever a meaningful fraction of sales? Or, phrased differently, is Microsoft actually leaving any money on the table by making Windows 10 upgrades (both the Windows 7/8 to 10 and 10 feature releases) free for consumers rather than paid?

But it's never enough. Gotta get that growth.

So? There are free products with ads (like Gmail), paid products without ads (like Netflix and Windows 8), and paid products with ads (like Hulu and Win10 home edition). I'm not the biggest fan of the paid+ads business model, but it's a valid model and Microsoft has been pretty consistent - Win10 has followed this model since release, so nobody is being tricked.

People were tricked when they were forced to upgrade through confusing prompts. People were forced when Microsoft strong-armed processor companies to not support Windows 8 and instead force Windows 10’upgrades if you wanted new machines.

Disclosure: I worked for Microsoft during Windows 8 through early Windows 10 development on related teams.

> Microsoft strong-armed processor companies to not support Windows 8 and instead force Windows 10’upgrades if you wanted new machines

Do you mean to say that Microsoft clearly communicated Win8 EOL dates and OEMs choose to move accordingly? If not, can you cite a source?

Edit: after some googling, I think you mean - Microsoft didn't think it was worth the expense to pay engineers to backport processor support to Win8. Bit of a difference from "strong-arming" the processor companies