It's guaranteed to have at least the terms of the permissive license (usually requiring attribution), but no, it does not guarantee code released under a permissive license will remain available under permissive terms. That is literally the point of the permissive terms: so people can apply more terms under a sublicense.
> From an end user perspective BSD/MIT and GPLvX licences offer the same guarantees.
No they don't. I can decide to stop distributing a BSD/MIT licensed application in both source and binary form, in favor of only distributing it in binary form under a sublicense. As a user, this is not "open source forever". This is "open source until we use the distinguishing feature of the license to make it not open source".
GPL, assuming multiple contributors and no CLA (both of which are extremely common) ensures this cannot legally happen (unless they somehow get all contributors to agree against their exercised rights).