We have a bigger problem and it's popularity. Some instances will want to become as big as possible, for resell value.
So they will be encouraged to let the maximum of people registering a new account, including bots and spammers. Because they make the numbers and they start the snowball effect.
Didn't you all looked a the most populated instances/magazine when you registered magz, and you didn't care about the mags with zero activity? Well, you are encouraging the process in a way.
People will be reluctant to defederate or to ban popular magazines from other subs. So the float of spammers is unlikely to stop, because the person who makes the decision, the admin of the instance, will welcome them. And you won't ban the instance or the mag.