I'm late to reply here but kind3 of a trusted npub is good enough to assume that an npub isn't a spammer which is all I care about at the relay level. Sort of a bare minimum trust implemention that can easily be improved but already does a lot for us when it comes to stopping spam
Discussion
This is a hot topic. Every time I post about it people have interesting takes. You are not alone in thinking “follow is good enough”. And still others have much more elaborate ideas for measuring trust. I should host some more panel discussions …