The registrars have an interest in not delivering requests for sale or even abuse reports, because it's likely that they will result in the domain either being decommissioned or moved to another registrar.
WHOIS Privacy Services have been doing exactly this for a long time and they are usually paid by the domain registrar.
I think that problem is more imagined since there doesn't seem to be this problem in the real world.
Yes, but crucially they are not the domain registrar, and that's because this was already a problem.
Honest and good registrars don't have a problem with WHOIS services, you claim they would since the emails they forward could lead to lost sales.
The WHOIS service is just as incentivized to filter out certain mails since they are paid by the registrar and thus get profit of sold domains.