The biggest source of incompatibility isn't in the programming languages. It's either in the specifications ("hmm maybe one byte isn't enough after all for a character, let's break all those assumptions") or in the hardware ("maybe 16 bits isn't enough of address space").
What do any of these have to do with guarantees of long-term compatibility? I'm not arguing that there should be One Programming Language To Rule Them All, I'm asking about whether we can design better guarantees about long-term compatibility into new programming languages.