won't spammers simply start using real addresses that just point to /dev/null instead of a legit account?

Yes, to a point. The premise behind these systems is to make it more expensive (and hence less economic) to send spam. Also, as soon as the spammer has to have an address to which mails can be sent (even if they are dropped on the floor), they add a degree of traceability.

The TMDA premise is that it requires a real account (and could require a real person, with a suitable challenge-response mechanism), which changes the economics still further.