Msft certainly don't turn a blind eye. I know this first-hand. It was part of my job as Sales and Marketing Director at Microsoft to actively pursue illegal and unauthorised channels and shut them down. In South Africa I personally founded and launched the local chapter of the BSA in 1992, along with Autodesk. We worked assiduously to go after and close illegal and unauthorized channels to a) protect IP and copyright, and b) support authorised distributors and their dealers.
Back then it was a lot easier to go after the people selling these keys directly. Maybe you remember the takedown of PE Technologies back in 2011? That's a much easier target than people who were selling illegal copies of Windows XP/Vista/7 on the streetside in the townships.
I liased with a local software and marketing rep for Microsoft in PE as well when I was there, and there was a lot of emphasis on not only selling licenses correctly, but also sourcing licenses properly (not through Sahara like my ex-boss, which I've always suspected used grey imports to undercut the other distributors).
Microsoft makes its money selling software and related services through the authorised channels. It would be utterly idiotic for Msft to turn a blind eye to illegal and unauthorised resellers who undermine its channel-centric business model. Every single IHV, OEM, aggregator, distributor and VAD on the planet knows that.
And today that's something that no longer works. Microsoft picks its battles with sellers that they know they can target. For MSDN/Brightspark/whatever resellers, they blacklist the activation keys. For people who are just using Windows loader to get a new copy of Windows 10, they accept that it's part of the market they're in and won't be a simple thing to police, so they accept the users who may turn into legitimate purchasers from the store or Microsoft's other services one day.
When I say they largely turn a blind eye to these things, I'm not joking (and, given your history, you're probably familiar with this as well). They never prosecuted even one percent of people running pirated copies of XP and 7 in China because they knew the approach to try get everyone on a legitimate license through legislation won't work. They'll target the dealers and resellers, but not private individuals. Different strategies need to be used in regions like China and Russia. They even partnered with the Chinese government to customise Windows 10 for their region and spying needs.
I suspect that Nadella was behind the switch to generating revenue through software sales and advertising all along, knowing how many sales the Office group lost to pirates each year, and seeing that the Windows division had the same general problem, but to a lesser extent. Office 365 was his baby.
Pretty sure they wouldn’t invalidate the license as that would affect their end user customers.
They would go after the sellers which I’ve seen happen aplenty because they never stick around for very long.
Actually, license blacklists happen in waves quite often. That's why resellers typically have some line that if you have a problem with a key, get back to them and they'll give you a new one. As Microsoft figures out which accounts are being used for reselling purposes (activations get an IP address assigned to them as well as geographic location), so they slowly blacklist those accounts.
Microsoft even knows exactly how many Windows 7 installations that have cracked keys are out there, because each and every one of them pings the mothership occasionally.
It's still an uphill battle for them. They couldn't target users in the r/hardwareswap subreddit, so they had the mods shut it down and install proper rules for software sales. As that happened, more popped up offering the same thing under a different name. Policing this properly is almost impossible without tying a Windows license to an active Microsoft account - and even then there are surely going to be ways around this.