For a medium-large registrar who may send out tens or hundreds of thousands of expiration warning messages per day, bounce processing and analysis is not really practical.
Actually, automated bounce processing is a pretty well solved problem, since we've had to figure out how to do it to manage mailing lists. The question is what to do once you've figured out that the address is bouncing. There's also the problem of abandoned addresses that don't bounce, but that nobody reads.
But the concept of telephoning or even sending paper mail is not necessarily unrealistic. Yes, there may be a cost involved (although not particularly large for automated voice messages within much of the developed world). How relevant that cost is depends on what percentage of domain names get to the point of expiring and still not renewed, saw a week or two later).
I agree that it's probably pretty small. It's also my impression that the majority of abandoned domains are abandoned deliberately so only a small fraction of the hard to contact domains even care. At some point, I think it's reasonable to expect people who want their domains to stick around to make a minimal effort to keep them around. What I don't know is how hard it would be to educate people that there's a reason for the WHOIS contacts, and "I don't like spam" doesn't override that. R's, John