Unless the conclusion after a set of premises is necessary or from a deductively valid argument, isn’t every inference technically a fallacy?
... and? You make that sound as if that is novel or surprising, it isn't. Of course pretty much all inferences in real life are fallacious. What do you expect. We work with incomplete data, with assumptions, with data that is only measurable to a certain accuracy and you expect all of that to yield "the complete, eternal, unchanging truth"?
And so if every inference is a fallacy, why the need for things like fallacies in the first place? There is no inference that logically follows.
Now it's important to realize what that means and what it does not mean.
As implied in the previous paragraph it means that what you infer is not "the truth". Any story that is incomplete can not be the truth and should not be treated as certainty. So if you do treat it as certainty you are committing a fallacy. And or must be aware of the fact that you're making an assumption (setting an axiom rather dealing with something that has been proven to be true).
Now the other important thing to realize is that, just because it is a fallacy does not mean that it is necessarily "certainly wrong" or that it is useless.
Like suppose you have a rigged coin that lands heads more often than tails, you watch a few rounds and it's something like 80:20. Suppose you infer from that, that it is better to bet on heads than on tails, then you're likely correct more often than not. But if you were to infer from that that it is certainly better to go for heads rather than tails you'd commit a fallacy. You could make idk 5 games and happen to be so unlucky to get a 5-tails streak. It's rare but possible. Making the inference is useful and it is reasonable given the circumstances, but it is not TRUE.
For example, appealing to popular opinion to defend an argument is often considered a fallacy. But clearly, there are many things we believe in based on popular opinion and yet don’t doubt and don’t consider them to be fallacies.
As said there is a difference between a heuristic and a truth, a truth has a definitive truth value so you can make deductive arguments with it, a heuristic just works most of the time and so you can make arguments that are likely to be true, but not always. Which is better than making arguments that are false all or most of the time.
Also there are different reason to appeal to popular opinion and not all of them are fallacious. Like if you ask what you should eat for dinner and pick the one that most people liked and that no one is allergic to, that would be a good appeal to popular opinion. While if you asked 100 people if they agree with you whether 1+1 is 3, that would be a fallacy. In the first case the opinion of the people was relevant to the question in the second it wasn't.
But if every inference (unless deductive, which is usually not an interesting inference) is invalid, why bother with the notion of fallacies? It is not as if certain non deductive arguments are better than others apriori. They all don’t follow.
No, just because all inferences in the real world are wrong does NOT mean they are equally wrong or that there is equality in their wrongness.
Also as others have already remarked you can still use deductive reasoning and make interesting inferences by making axiomatic assumptions, "what if X were true?" Like it's a practice that is quite successfully used in math which has contributed massively to the development of our sciences so to cast that off as uninteresting is quite wrong. Now does saying that something is true, make it true in the real world? No. But it's a method to check the self-consistency of a statement, so if it fails in that regard it's already worse than one that doesn't.