Both LuaTeX and XeTeX have the original TeX code at their core, so they are not bottom-up rewrites. The purpose and benefit of backward compatibility for TeX i

Both LuaTeX and XeTeX have the original TeX code at their core, so they are not bottom-up rewrites. The purpose and benefit of backward compatibility for TeX is that a 30 year old manuscript written in TeX can be fed to the latest version and produce identical output to what was produced 30 years ago. (Of course, you'd also have to archive the macro packages and font files you were using, but those were never machine dependent).