(No relation to Microsoft, just a general observation) As far as we know, Microsoft has not categorically denied that they will create another operating system in the future. It'd be terribly stupid for them to do so.
The free for the first year has caveats but there is no scenario where we are discussing Microsoft making you pay for essential updates in the second year. In fact, while I am not a Microsoft fan I will stick my neck out and say that it is extremely unlikely that you will have to pay for updates in consumer SKU's of Microsoft Windows for the next six years. Seven years is typically the regularly advertised end of life and end of mainline support for previous versions of Microsoft Windows.
The first major caveat is that this is for existing Windows users. This offer only applies for people who currently on qualifying versions of Microsoft Windows today. If your computer came with a genuine copy of Microsoft Windows 7 or above, it is in Microsoft's best interest that you get on board with the latest version of Windows.
The second major caveat is that this is specifically not for organizations. Enterprise cash cows will remain cash cows.
I'd urge you to be skeptical and not infer any promises Microsoft has not specifically made. However, if anyone has been telling you that Microsoft will make you pay for security updates or Windows in general beginning the second year are at best incorrect and at worst maliciously spreading FUD. Of course, Microsoft isn't helping because there are so many details we don't know yet such as when is the end of life for Windows 10 (assuming I accept all updates)? There is a lot of things we don't know. However, we can say with a lot of confidence that you will not have to pay a subscription the second year of moving to Windows 10.
For what it is worth, I intend to update to Windows 10 on my personal computer as soon as possible. I also intend to stay on the fast lane of updates (more updates more often but possibly some buggy updates) once I get on Windows 10.