All the insider builds have an expiration date to then be replace by another later build.
The Upgrade to 10 from 7, 8, or 8.1 until July 29, 2016 was a year long offer to get 10 free as an upgrade to the existing OS. That time has now come and passed. Even to get Insider build activated initially you had to see at first an upgrade over a previous version until later build during last fall then saw the option for entering a valid product key from a previous version.
If you are only now looking to see 10 running to replace a previous version you will either need to buy a 10 key from the MS online store or buy one of the 10 media options available often still found on optical while also seen on usb flash drive as far as removable media goes. Download of the Redstone 1 Anniversary update in ISO form will still require purchase of a 10 key.
You do not get Windows 10 for free. The insider program permits owners of Windows 10 to try out pre-release versions of the software. If you go to Settings/Update and Security/Activation you can see whether or not your Windows is activated. The expiration of build 14905 in October does not affect that. It means that build 14905 will stop working after October. There is a difference between Windows activation and specific builds expiring. Since you are keeping up with new builds you will keep getting new ones with later expiration dates. Preview builds are buggy (the whole point of the insider program is to find these bugs) and Microsoft does not want folks using buggy software beyond its usefulness as a bugswatting effort.
Not now you won't! A year ago until July 29th of this year being almost a full past now the upgrade offer was made free for those running 7, 8, or 8.1 machines. That free upgrade to 10 offer is no more! The Windows Insider Program being the correct full name is simply the option to try out as Cbarnhorst has politely commented on the developer not full retail builds being those used by developers more so even then beta testers in order to work out software bugs while the beta tester report Windows related bugs.
For the developer type Insider builds those definitely see the DOA date of the longest being six months before demise occurs where the OS shuts down completely which if not having been replaced by a newer build leaves a system void of any working OS unless having seen a dual or multi-OS setup from the start. When first looking at 10 back in January 2015 for the fast view at the time that was seen on a VM only.
As for upgrades that either have become unactivated or for problems seeing a fresh clean install not activate after 10 has already seen a valid activation you would want to refer to the Activation Troubleshooter. Activation Troubleshooter - Use in Windows 10 - Windows 10 Forums