According to Microsoft Windows Insiders are allowed to continue testing of Windows 10 Preview as long as they like. After the RTM-version of Windows 10 is published Preview is no longer there. However Insiders will take part in continuous testing of Windows 10. In a way there will always be a preview version available.
I feel like I'm a puppet on a string. Microsoft gets all the information and in turn my PC is running an OS with a lot of bugs. In addition the next Windows (after Windows 10) will be charged monthtly like Office 365. Insiders are the ones that Microsoft needs to get through their testing cycles what comes to the future versions of Windows. I feel bad. I don't want to pay for Windows every month and don't want to test that sort of software.
My prediction is there will be less Windows users in the future than there is now. People have less money to spend and they don't want pay for Windows every month. Selling operating systems WAS a good business for a while, not anymore.