Fine, show us where in the BRB are the rules for disembarking for failing a Ld test.
You can't because it is a hole in the rules.
It's not addressed at all.
Both sides of this have house rules or crap we make up.
My idea is no morale tests when embarked just means we ignore the test for 25% loss, just like we do when locked in combat.
Your idea means we have to come up with a ruling for disembarking, is it allowed or not.
Then if they can't disembark are they destroyed or not?
Your idea adds complexity, and the need to make up even more crap in order to make it work, for no good purpose.
Keep it simple, just ignore the test.