The problem about "Primarily Opinion-Based" is that the criteria for what defines POB depend on the person casting the vote - it's a big blurry region
The problems I specifically see with this question is that it states the following criteria:
- organism: "an individual entity that exhibits the properties of life" where properties of life would be things like being composed of one or more cells, being able to grow, being able to reproduce, ...
- physically infinite: we are getting to problems with being able to judge an infinitely large organism already
- lived for an infinite time: there are some creatures that could theoretically be classified as immortal, so this point seems to be somewhat valid, though there are still questions about diseases and such
- not "too vulnerable": what exactly is "too vulnerable"? The question states that it should not be "inifinitely small", but how small is too small and how big is big enough? What kinds of threats are there that this creature must survive? The question states that it would be vulnerable to attack, but what kind of attack would be viewed as critical for this infinitely large creature?
- room for people, not cutting off parts of the whole infinite space... my knowledge of different definitions of infinity is not good enough to talk about this stuff, but if there are mathematical tricks involved that challenge certain definitions then we are getting into problems with the normal definitions for "organism" again
- should have only one head: the currently highest voted answer uses fungus as a basis and nobody downvoted it for a lack of one of the criteria; or we have answers that go with looks like a head instead of a real biological head; there are some good discussions about this specific problem, but some answers seem to be extremely vague and not quite fulfilling the requirements of for example "not infinitely small", but there are also purely mathematical answers that don't seem to have anything to do with defining "organism" or "living" or "head" and sometimes people just more or less ignore that having one head is a requirement
- "I am going to be a bit lax with the physics of this world to allow for some basic functionalities" -> this basically tells me that everything goes because we are talking about mathematical definitions of infinity without regard for physics, but with the goal of having a biological organism that the OP defined as having lived infinitely long with an infinite size and exactly one head while not being too vulnerable
This question looks to me like a philosophical discussion where each answer should provide its own set of definitions to make sure their version works. This wouldn't be a problem, but the OP at the same time has some criteria that apply to normal organisms.
I recently created a list with some criteria for how I use close reasons, so let's go through them to see if something applies:
- "Too Broad" - "Writing a whole book for you is too much. I know what you want and I think it's theoretically on-topic from a content perspective (I can see that you are currently building your own fictional world), but it would take far too long to talk about all the details that are necessary to sufficiently answer your question. I need some more details so that I know what you want to focus on first - you can ask more questions later where you incorporate the details from previous questions. As the help center states: If you can imagine an entire book that answers your question, you’re asking too much."
There could probably be books about such a topic, as I feel like it's mostly a philosophical discussion, but the reason doesn't seem to fit. Writing a whole book to answer the question would be overkill even if it could be taken as the basis of a book.
Asking for creature-design is valid and talking about stuff like immortality is fine, but I think that the answerer has to define all things again, like "What is a head?" and "What is an organism?". Every reasonable answer should provide such definitions or at least mention which definitions they are using. After that each answer seems to equally valid.
POB fits in my eyes.
If the OP pointed to clear definitions for:
I could be convinced that the question is not POB, but currently it is. And the different answers that sometimes ignore some criteria while still being upvoted and not discussed as not fitting are an indicator of this problem.
- "Unclear what you are asking" - "I have no idea what this is supposed to say. Either there is a big language barrier, the querent has misunderstood quite a few things about the topic he is asking about, there is no question or there are so many things missing from the description of the world that it feels to me like a random excerpt where too much is missing to make any meaningful attempts at answering the question. I don't know the situation, I don't know the goal - I just don't know how to help the querent yet."
I understand what the OP wants, there is no language barrier - the question is clear. One could argue that missing definitions should make me say that this question is unclear, but I can see what the OP is trying to achieve so I wouldn't vote as unclear, but as POB.
- "Off-Topic" - "This is not about WorldBuilding (as defined in our help center). You are for example asking about help with your story, about character actions, or about something completely different, like misunderstanding that there are multiple sites on the network and accidentally posting a question about programming languages on WorldBuilding, ... We can't help you here, you need to go to a different site on the network or for example to a forum to find the answer you are looking for."
We can't help you with open-ended discussions, but we could definitely talk about immortal or incredibly large creatures and I can definitely see the worldbuilding, so this doesn't apply.
I voted POB on the basis that I can't see how answers can be rated against each other if everyone is supposed to use his own definitions for the criteria presented.
We are not mind readers. We can't know the definition the OP uses, but we need to know these definitions to rate answers against each other.