While a support policy for earlier versions is a good thing to have for software at a certain level of maturity, it is important to be mindful of the trade-offs involved. Specifically, supporting earlier versions uses work hours from skilled contributors that could be used elsewhere, possibly to improve the language.
Regarding “serious” use: I am not sure that a support policy for earlier versions was a key factor for all languages which became popular. I am under the impression that some languages became popular first, then built up a large community which had the resources for backporting and security fixes.