Spam filtering has been a basic relay activity since day 1. I don't think adding spam filtering moves us to smart relays, dumb clients. 

Also, the reason I don't see much spam is because Amethyst also has spam filtering. So, it's not that clients are giving up. 

In the end, clients alone can't solve this. Relays alone also can't solve it. Both need to be developed to make things work.