If the registrar is in the middle, they have a considerable interest in failing to deliver sales inquiries, because they will tend to lose out on them. Same with certain types of abuse inquiries. The registrar has a conflict of interest with most forms of legitimate inquiry on a domain. If the registrar allows unfiltered messages, or messages whose content is entirely unknown to the registrar, then you're back to square one, because the end address ultimately has to filter the spam.
Simply write delivery into the contract, and require fees paid to the registrar from any party sending mail through the service (i.e. stamps).
Critically, if one were going to do this, allow the domain owner to black hole all communication via this path if they're not interested.
But I do see your point. On the one hand, I'd suggest an optional "reach the owner" part of whois, where one could list a physical or digital address (doesn't even have to be email, I sometimes list social network presence on "about" pages as contact method for unimportant sites, to prevent spam). But on the other hand, we already have a place where you can optionally put your contact details: the very website we're talking about. And there you can apply any anti-spam measures you want, which you cannot do if it's in a public directory somewhere.
I think it's fine if people have to publish their contact info themselves.