I'm not avoiding your question - just stating that it is irrelevant.
Say the answer is 'yes' for arguments sake. Landlords can afford to lower their rents (which assumes every landlord is in the same position which is of course an easily rejected assumption). A rent cap within the 'affordable' level will reduce the landlord's income in order to reduce their tenants costs. But not to an extent where supply or quality is necessarily impacted.
Or let's say 'no' for arguments sake. Landlords cannot afford to lower their rents (an assumption which is as questionable as it was before). A rent cap will result in diminished supply and quality of private rented homes, as well as being detrimental to private landlords.
These are pretty much what a yes or no means in terms of superficial outcomes. Superficially, you might say if the answer is yes, then as the landlord is not harmed, then it is OK.
Regardless of a yes or no, in each case the imposition of a rent cap is still unethical for the reasons I gave before. The landlord should not be forced to give up his earnings because of the position his tenant is in, whether he can afford to or not does not factor.
Rick seems to get the principle of it.