How will new feature development be handled with Windows 10?

Microsoft say that Windows 10 will be the final version of Windows. Whether that's true or not, it certainly won't be the end of new features being needed, wanted, and added to Windows 10.

That being said, the "feature complete" state of Windows 10 could probably be viewed as Windows 10's starting point, rather than its finalization. And in 5 - 10 years, Windows 10 could have a very different presentation than it will at launch - and that seems very plausible, with how fast new technologies, devices, and types of services are arising.

Will Microsoft release updates via modules that people can choose based on their relevance to them, and will they be free for Windows 10 owners?

much like Apple does ,and same as Apple there will likely come a time when your hardware is no longer compatible with the latest version of win10 ,[when ever that happens ],, my older mac laptop is no longer updatable ,because latest mac OS no longer supports my intel duel core cpu ,so I'm stuck at 6.8,,and cant go to 7x

Well that sounds terrible. But that's a whole new version of the OS, so for Windows, it would go from 10 -> 11. Even if it wasn't called that, such a foundational change would be a new iteration as much as 95 to 98 was, or Vista to 7, 7 to 8.1, or 8.1 to 10.

It's terrible that a new OS would not support certain CPU model if it has nothing to do with performance capability to run the OS. There are no restrictions on what Windows version PC hardware can run, so far - but if a new version of Windows were designed to not work with certain generation of CPU, despite those CPUs having enough power to handle the OS, that would be terrible. I have trouble imagining that a dual core CPU could not handle any modern OS. It sounds like the lack of support caperjack is talking about is a software design, and not a hardware weakness.