What opportunity is being lost out on because of space exploration?

Whatever you can imagine they could spend the money on, including leaving it with the tax payer or taking on less debt.

(And, if you don't like the monetary framing: just look at the real resources spend instead.)

However I'm not nearly as harsh on unmanned space exploration.

	▲	fastball an hour ago \| parent [-]
		That's not how resources work. Resources that are used for space exploration aren't magically available for anything else when you don't do space exploration. The economy is not a zero sum game and human capital is not fungible. A rocket scientist/engineer/technician/etc at NASA is not going to work on the thing we "should" spend money on instead if tomorrow you shut down NASA's manned spaceflight programs. They'll probably go work on ads at Meta instead.