Does anyone know if older SCA versions will reach an end of life where you will be forced to upgrade? Has this happened with any versions yet?

No one know this, no. We still permit customers to use Site Builder if they really want. But generally speaking, the older your site code gets the more creaks and cracks you'll hear and see. Businesses should be mindful and have a strategy in place to keep their site up to date.

Yes we are looking at future web spending and making plans. We have a heavily customized SCA instance so moving is not an easy task.