Reveal some nasty unpatched vulnerabilities after the support end :D
I think the only thing Microsoft can do is educate (seminars, through partners) and i think this is already happening and most admins know that. Maybe some discounts for upgrades, at least in the public sector.
We have used 2003 Server 1 or 2 years past support because hardware we owned only worked with that version and we didn't have a budget to replace that hardware. When i was leaving my last place they had around ten VMs with 2008/2008R2. Last time i spoke with them a few months ago they still had at least one of them running. Some should be easier to move, but for some i just can't imagine them ever doing that. One farm with 6 servers of highly customized SP2010, which is a huge task to even port to 2013/2016 (not even mentioning SPO). And this is the main LOB app. One 2008 server with the main finance management app made my a not so cooperative local provider with 10+ years of customization, libraries, frameworks, etc. And it can't be stopped for a day or so. And one somewhat new admin and another one completely new.