That changes nothing. I can't receive unsolicited messages on my xmpp and matrix accounts until I allow the contact invitation.
At worst you could be flooded with invites but that wouldn't be much different than any one of our linkedin accounts and there could be easy mechanism to ban external servers that are reported to allow too many spammers.
I'll grant you that - but the proposed legislation isn't requiring Apple to allow iMessage users to receive unsolicited messages from external services.
It would require additional effort on Apple's part to keep third party tools' spam efforts at bay. At some point the cost of doing that isn't something Apple could justify, and at that time unsolicited messages absolutely will increase in volume.
If anyone can do iMessage on any device, it is logical that these protections may weaken over time.