Questions with multiple answers are not inherently off-topic
We are getting into The problem with opinion based questions here. The main problem from my point of view is that this is a big, blurry region where on-topicness and off-topicness are close to each other and everyone has a different understanding of "where to draw the line" and where everyone has different tags where he may be more or less lenient.
There should be a way to somewhat objectively compare answers to each other.
This can be about internal consistency, about how well it fits to the setting described in the question and about any number of things. We don't need exactly one correct answer for every question - but there needs to be a way to find the best answer, even if all answers would fit the bill.
What we don't want is a question where every answer is exactly equally valid. If there is no way to say that one answer is better than any other answer, not even because "it fits better to the setting described in the question" or similar things, then the question is "primarily opinion based" and should be closed as such.
This site is more opinion based than many other sites on the network. We have to live with this and have discussions about on-topic/off-topic, especially with a focus on "pimarily opinion based", from time to time, because the network is not exactly made for opinion based questions. That's why we need to help whenever we can to make a question somewhat objectively answerable. This, in turn, is opinion based - which makes this whole topic very, very difficult. What is somehwat objectively answerable?
We can't give completely general guidelines that are always applicable - we can only try to give some rules and every once in a while there will be a discussion. The community will then decide on the fate of the question on a case-by-case basis. How the community exactly votes will shift again and again and again - what has been on-topic a year or two ago might not be on-topic anymore. And what is off-topic right now might be on-topic again in a year or two. Nobody knows.
The current guideline is that questions are not "primarily opinion based", just because there are multiple valid answers - questions are "primarily opinion based" if every possible somewhat reasonable answer is exactly equally valid.