Is it acceptable to say that a reviewer's concern is not going to be addressed because then the paper would be too long?

This is the kind of thing you ask the editor about. "If I include this philosopher's view, the manuscript's length will increase by roughly 40%. Are you sure?" If the editor says yes, you're in the clear - they are clearly going to waive the word limit. If the editor says nevermind, you're also in the clear - even if the reviewer objects, the editor's on your side and can overrule the reviewer's objections if the reviewer recommends reject.

You can ask the editor for advice, but you probably can't force your will on any journal. Ignoring reviewer advice is risky. Going over the page/word limit is risky.

So, acceptable, yes. But it may not result in an outcome you would favor. Only the editor can say what will be accepted when all is done.

An alternative, perhaps, depending on what you have, is that the original together with the reviewers suggestion is enough material for two papers if things are moved around a bit. I can't judge that, of course, but it might be worth a look.

This largely depends on if you think that addition is useful or valid or if you think that the addition is unneeded or will detract from the focus of the paper.

If you think that the addition is valid and useful, one way forward is to reduce words elsewhere: cut was is less important, restructure, and be more concise to fit this new information in. You can also contact the editor directly and ask about additional words. If the journal still produces print versions you might be bound by the stated word limits. If they've gone entirely digital, there are no physical constraints so it is just what the editor will allow. Usually there's a bit of wiggle room for articles post review and many of them must grow a bit.

If you do not think the addition is useful and valid, then the fact that inclusion will significantly increase the length of your paper can be put in the response letter as a reason that you will not be taking on board that particular suggestion. If you are going to do this, do think of a way you can nod to this philosopher/corpus of research/whatever in a sentence or two, if only to explain that they will not be discussed here but further work to include them in the analysis could be interesting in the future. That often gives the editor what they need to allow your paper to continue. This won't always work, especially if the reviewer said that your paper is invalid without the addition, but if it was more of a suggestion than a mandate it tends to be enough.

Good Luck!