The relationship attestation isn't my concern. My concern is that now clients need to keep track not of one key but of many. You could use a different sub key for each client you use, so your profile might refer to more than one key at a time cause you use 3 clients. Additionally you might rotate keys even without knowing them to be compromised, so for older posts you would need to track additional keys.

Currently, relays already reject queries for long follows lists. Multiply those lists by 10 and you might see the issue.