It gets better, even if you change the MX entries, there is no guarantee that GMail will deliver mail to those new servers because the domain may still exist in their system. So you may remian cut off from a significant number of people.
Obviously any email sent in the interim will be lost, but any email delivered after the MX record change should just be sent to the new provider.
Based on the quality of other Google products, I think it's safer to assume the inverse: Google's mail servers likely know about a domain's MX records and adjust accordingly without account configuration or external intervention.
Maintaining a system as large as the one they do without this feature would seriously suck for the mail admin - it really has nothing to do with the customer at that point. I wouldn't want to work that job if it did because of a) all the angry people it affected and b) the overhead in my job introduced by a bad programmer.
This is a fact seen in practice. You can check it if you want. Just create Google Apps account and configure mail without setting any MX records in your DNS. You will be able to send and receive emails to and from that account and regular Gmail account.
I feel reasonably confident that ignoring MX records would violate some part of the smtp process, and I would be be very surprised if Gmail did this.
I had troubles a few years back with e-mail routing from GMail to mail servers for a domain that had previously Google Apps set up for it but was subsequently moved to another provider. They disappeared after deleting Google Apps.
Maybe it's not an issue any more, maybe it was just some weird caching problem, maybe disabled domains are handled differently but it wouldn't also surprise me if they had some system for handling what is essentially mail internal to GMail.
Thinks might have changed in the meantime, but my guess is that Google still sends emails directly for accounts that it "knows" are hosted on gmail and apps and happily ignore MX records.