Forwards and backwards stability. I want my language to be able to run code I write today in 2035. And for code I write in 2035 to work on my toolchain from today. I don't want code I've written to stop working or only work on eternal wave of newest and break on setups a few years old. I don't want to install a separate version of my language for every application, I want to use the system version. I don't need the latest and greatest computer science features. I just need something that can do almost everything and keep doing it reliably forever* (*in computer tech terms 10yr+). It does exist but it's rare.