It's not merging accounts as they seem to think it is. I don't gain access to my partners data just because we are on a family plan. Really the only "issue" we have is the credit card has to be the same but that's such a small issue to resolve since we have a shared credit card for shared expenses.
I don't really see the issue with going the route of just setting up a family link between your separate accounts if you have a dog together. Honestly that feels like more of a commitment...
Regardless of that for a moment. I think its better that the only way you can "share" a AirTag is through family. I can't really think of a reason that this would be an issue with friends that can't be easily solved by the 24 hour notice (or worst case needing to do that for a few days if you are doing a trip together or something).
Adding the ability to permanently do this otherwise I would worry could either be accidentally enabled or have a strong risk for abuse.