Linguistically speaking a common word like beer has no reason not to have various senses, unless it's being used as a technical term. Philosophically speaking a technical term is just a specific sense of a word that is more strictly defined in order to express a concept. A technical term is as specific as necessary in order to convey the level of detail the technical user requires, and for this reason different fields can have different technical definitions of the same word.
For example a dining table could be anything you put stuff on when eating, or it could be anything that is manufactured to put stuff on. In different circumstances you could define it either way depending on what the goal of your inquiry into the matter of whether something were a "dining table" was.
Like McMullan says, altering the ratios and stuff can easily invalidate the definition of a product, so the ingredient list is not sufficient to declare two things equivalent. Adjunct is a good example. half a pound of flaked wheat in a 10 lb grain bill is beer, but 8 pounds of flaked wheat in a 10 lb grain bill, most people would not consider that beer I think, so you could say that the ridiculous amounts of hops in some NEIPAs could disqualify them from being beer, if not having that much hops is important to how you define beer, that is, if you have a practical need to distinguish "NEIPA" from "beer".
A lot of the time a term is accidentally integrating a bunch of separable but related things with varying degrees of separability. For example you can say someone is "Japanese" and you probably mean both ethnically and culturally (as well as speaking the language), but you could rarely mean just one of those, whereas if you're talking about someone being "Italian" in an American context anyways its relatively more likely that you only meant ethnically even though "Japanese" and "Italian" are both ethnonyms with analogous structure and derivation.
In this vein, the vast majority of malt liquors are beers, the vast majority of ales are beers, the vast majority of beers contain mostly or exclusively barley etc. There was a time where in the US "beer" probably meant a lager, but we don't say that "ales aren't lagers", and even the Germans have Hefeweizen, which is the OG hazy and fruity beer and I have never heard anyone say "Hefeweizen is not beer". IMO, as for example, porter, hefeweizen, amber, pale ale, Budweiser, and pilsner are all quite different, so asking someone to "give me a beer" if you are picky would not be a good idea in much of the anglosphere or Central Europe.
I don't see a practical reason why NEIPA isn't a beer, it may have a lot in common with beer cocktails and other "flavored" stuff, but it also has a lot in common with beer. It comes in a beer can (pre-mixed as it were) and unlike say flavored malt liquor or wine coolers is not cheap or disgusting. Personally I am not picky so I would be fine with asking someone "please get me a beer" and wouldn't complain if it happened to be a WCIPA or a Hefeweisen, or a NEIPA, although I may be cross if it was a super crazy sour. For those of you who feel otherwise, perhaps we need a term to describe "regular beer". "Barley beer" would mean in the sense of not being wheat, "Reinheitsgebot" would mean in the sense of having zero adjunct in addition, and you could think of other things like "pre-NEIPA" or "boil hopped" to distinguish ones without a ridiculous amount of dry-hops. Or for those who hate everything hipster, "non-ipa" may suffice.
Like I said before, what REALLY sucks is that "IPA" is such a watered down term. I don't know why you should not be able to say to the waiter "I'd like the IPA please" without having to break out a style guide.
I think it also bears pointing out that in antiquity most beer was flavored, and the way they made it was totally different, they made it out of bread, although it also was not called beer, the Latins didn't have a consistent term for beer in antiquity instead referring with different terms to all the various forms out there much as mostly non-malt rice liquors are referred to in English separately as sake (nihonshu) and makkgeolli and not together as any consistent term (such as rice liquor). Along the same lines, you could malt up some nice malted corn and call it "Chicha" or "corn beer", and nobody would question you, since chicha is the only major kind of corn beer, but if you served it to someone from Peru they would be liable to remark "esto no es chicha!".
Generally, people should use as specific a term as they need, bearing in mind where they are when using it and what the people there would assume they mean by it. But what's probably more important here is to consider why someone may care about these matters.
It is not generally well received to tell someone they aren't allowed to like what they like merely because it diverges from what other people are doing, if it has nothing else bad about it, however it's also the case that going into an established field and re-defining terms and telling people they have to accept the new ones is not very well received either. In such situations its best for people to add a bit of nuance to their language so that both the purists and the non-purists can make themselves understood.
When I said it was lawnmower beer, all I meant was that it was suitable for swilling down in a hurry on a hot day.
Here we see "lawnmower with regard to ABV" contrasted with "lawnmower with regard to drinkability".