Well, the Duo will be supported for another year and the Duo 2 for another two. Neither was destined to sell in a large number, and they were kind of ahead of the Android game as it's only with Android 12L that the operating system can do what is required for dual screens.
Will there actually be a 3rd version? Well, they are working on it. Will the third version be dual screens or a folding single? Who knows. Personally, the dual screen second device works much better than a single folding screen as I can be much more productive. Remember that although you can use the Duo and Duo 2 as you only phone, they were designed as companion devices - in other words you carry two phones!
I'll just address the comment about updates. Microsoft has done a superb job with updating the Duo for security patches and bug fixes. OS updates have been delayed until they got it "right", and given how much is still wrong in 12L I'm pleased they waited.