"I think we need to be honest with ourselves and just admit that there is really nothing in space so valuable that it would justify the expense of going out to recover it with our present level of technology. We would need at least an order of magnitude or two improvement in propulsion, not to mention substantial advances in radiation protection, artificial gravity, and life support."

I would agree with the idea that NASA going into space to mine anything will never be worth the cost of doing it.

But there is already something in space which is valuable that justify the cost of getting to it: high ground.

Satellites monitoring the weather has saved lives and saved billions of dollars. There is no question that they worth every penny spent.
Direct TV seems fairly profitable.
Using satellites to rely communication of all kinds it also been profitable.
Military satellites may have prevented a nuclear war or two, and they have saved money and lives in terms trying to somehow get anywhere near the quality of information one can from satellite.
Etc.

Space isn't an option, it's needed- assuming you don't prefer to waste enormous amounts of money and lives.

"There is plenty of science worth doing out there, but it can be done by robots, and science may not yield a direct payoff on investment for decades, even centuries."

Or they could seen as having an immediate return- some people find it interesting or entertaining.
You could say it is unique governmentally provided entertainment for the enlightened.
And considering how poorly NASA tends to keep it's records, it's doubtful your idea has as much merit as mine.

"I can only think of two things which would financially justify an all out effort in space at this time: either the discovery of an alien ecology somewhere else in the solar system with an alternative biochemistry to explore and exploit industrially, or a cache of extra-terrestrial technology, perhaps a millenium-old Bracewell Probe, preferably intact. Both are long shots, to say the least."

Well, something like a sample return could handle these things.

And nothing in space would justify "an all out effort"- it's Napoleon invading Russia. Or putting a million troops in Afghanistan.

It's simply the wrong approach.

"There is only one other reason to go now, and it is not a commercial venture: Planetary defense from comet or asteroid strikes; but only when we have the technology to do something about it."

There is nothing particularly difficult in terms of the technology. It's about delivering energy to a location in space. The energy could be in the form of a rock. You would need to to be able to find a small rock and move that rock to the proper location to hit the other rock.

But if that seems too complicated, you could use a nuke.

Practicing using nukes on various kinds of space rocks and seeing if the predictions match the results could be a prudent idea- but probably won't fly, politically.

Perhaps, those of the less enlightened public might find rocks crashing into other rocks as somewhat amusing.