(you can make a data structure which makes memory explode, for instance lists:seq(1, 123456789123456), you can make a program which makes memory explode, for instance by growing the stack enough. Why is a message queue special?)
What happens if the message you can't add is a specific system message that allows the suspension of OTP processes for a code upgrade? Do you end up livelocked? How do you propagate back that you couldn't insert your element in the queue? Do you make message passing an operation that can fail? How would that work over the network when the message telling you something failed is lost over a netsplit? Is it a different kind of failure?
This has very deep design repercussions on the language. It's not impossible, but it wouldn't let Erlang be the same.