In fact, western governments can't give such informal guarantees further than their election term even if they wanted to, because unlike in Russia, governments change every 4-5 years and the current president, prime minister or cabinet ministers can't promise what their successors will or won't do, because the successors are often completely different people from different political parties with vastly different political platforms. That's why we have written treaties. Soviet and Russian diplomats are without any doubt educated enough to know that.