One benefit of making a 'properly new' language is that you don't have to support old code; I've definitely seen a lot of cool optimizations dropped because whi

One benefit of making a 'properly new' language is that you don't have to support old code; I've definitely seen a lot of cool optimizations dropped because while they'd help in one place, they'd blow up a loop somewhere else, and ultimately there's only so far you can go with using heuristics to make your optimization apply to one but not the other. Sometimes (e.g. taking advantage of UB) this can even be functional -- 'breaking' some pieces of non-conformant code in exchange for getting better performance elsewhere. This is especially relevant in broad-domain languages like C/C++; Linus in particular has opined multiple times against the GCC devs choosing to implement new optimizations that ended up breaking code which had been working fine in Linux for years up to that point.