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.
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.
So ideally, in case they don't provide an email, one would not do that.
I get offers for my domains, all unwanted.
If I at some point decide to sell one of those to one of them, all the cold approaches from the past don't suddenly get legitimized.
edit: as I'm writing this, I remember that this is quite similar to the selling of social media IDs. Some random crypto start up decided that they should try to bully me to sell them my three letter Twitter handle. Really annoying.
My state department of motor vehicles maintains a database of vehicles registered in the state and the contact information for their owners. While they do not have a convenient website for querying this data themselves, they make it available to third parties in bulk, and I can obtain individual records for those third parties convieniently using their websites.
Both these databases are mandated by my state legislature. So it seems the same reasoning does apply.
Another difference is that this contact information does not, AFAIK, include recipient-paid [1] contact methods such as mobile phone (including text) and e-mail. Also, methods such as phone are generally considered far more intrusive that, for example, paper mail.
[1] nowadays this is mosty nominal, but we're not quite at zero
Not true. Many people don't realize the value of what they have. And/or they won't see a message because of spam etc (could argue that this happens with whois also but generally the more clues the better).
Note that with physical real estate it is often purchased by someone wanting a property that has no sale sign. Also (per my other comment) there are many people who don't know the value of what they have.
So it's not a signal at all in many cases (I have over 2 decades of first hand knowledge of this btw.)
Besides it all comes down to price. You may not want to sell your house but if someone comes to offer you 3x what you paid you might listen to them. You may 'not want to sell' for what the market price is. Selling prices for domains are all over the map.