"Either a proposition or its negation" is the logical principle of the excluded middle, which is not a fallacy.

The fallacy of the excluded middle is considering only a small subset of possibilities. It applies here, not with what was explicitly stated, but an underlying assumed view. It is assumed that arguing against spending current amount X of money on science, means arguing for not spending any at all. Obviously that would be bad, because we wouldn't have the current benefits. In fact, we can also spend somewhere in between 0 and X (e.g. X/2) or even more than X. We'll get some non-zero amount of benefits for these, and it's not as clear that it's worse off, unlike spending nothing.