I'm sure this will get blown up, but every streaming service works the same way. Everyone except Amazon and Apple won't even let you stream more than one song per account on a family plan. I doubt the contracts with the record companies allow this sort of thing on an individual account.
I HIGHLY doubt this is a ploy to increase revenue - the amount of people that own HomePods on individual AM plans and regularly stream different songs to different devices simultaneously (enough to the point that they would pay an extra $60-$80/yr for the privilege) is probably nearly nonexistant. I think it's a lot more likely that a record company noticed and asked them to fix it, or Apple wanted to get ahead of it before they pissed off their partners.
I prefer to think of it as a nice benefit for the time it was there.
You are most likely spot-on correct.
I will throw out a potentially common scenario, though, that makes this less of a 'good' conclusion: Consider I have a HomePod that my wife and I use. Only one of us cares about Apple Music (me), but we both enjoy the HomePod. Now consider that I go out of town on business... and, of course, take my phone with me. Apple is now telling me that my wife and I can't both stream.
Good? Nope. Bad? Perhaps. Annoying? Absolutely.
Now, for the record, I don't own a HomePod. I've invested in the far superior performing (though less musically appealing) Amazon ecosystem. My subscription to Amazon Music let's us address that exact scenario.
Comment to Apple: I can appreciate the nuance of subscriptions and do not expect things for free. However, given your high-price barrier to the Apple ecosystem, I do expect things to work. In the scenario above, if my Apple ID is primary on 3 devices - which I've paid $$$ for, they are MY devices and I expect to be able to play music on all of them. If my wife wants to also play music on HER devices, I need to subscribe to a family plan. Simple.
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Think about what you just said. That would be the textbook reason for calling it a Family plan.
That would be true IF the HomePod recognized users by voice and switched Apple ID's as needed.