> technically it's the same with proper RPis, where they're the only SBC board (not x86 baed) that you're guaranteed will have at least some software support after 5+ years

This isn't quite true, though it's true at the RPi's price point. For example, Toradex (disclosure: I work for Toradex) produces the colibri-imx6 system-on-module that can be turned into an SBC with a stock carrier board. It was introduced in 2014, continues to be supported today, and will continue to be supported until 2036. But that level of long-term support and maintenance costs money, and the pricing reflects that.