Still, what makes you think building a Hubble equivalent that would last 8-10 years is outside the realm of the reasonable when Hubble is expected to last 7 or 8 years after the last servicing mission.
Either way, would launching three (or six or whatever) of those on rockets really have been cheaper than the Hubble in terms of science/dollar? I have some serious doubts. Throw in the politics and overhead of three major missions, and you're talking big numbers.
And that ignores the intrinsic value of manned missions for their own sake - you must do them to get good at them. There are people who say that we should never do a manned mission, and that all the data we want to collect can be had by unmanned satellites and whatnot. While it may be very close to true, that's a very short-sighted point of view in my opinion.
I try to judge past decisions in context not just based on what worked. At the time we did not know that each of those servicing missions would work or even if the initial launch would work. Though, clearly one path worked and the Servicing missions where incredibly valuable experience. But, they could have also cost a shuttle and several astronauts lives. Granted, the odds of multiple telescopes being sent up in a fairly short time period is tiny so it's only reasonable on a technical level.
In the end I think the Hubble program is completely justifiable, but that justification needs to be about more than just a quest for pretty false color pictures or worse an example of the sunk cost fallacy.
Given that we have no shortage of astronauts willing to accept that, and that we do put a ton of effort into their safety, I have no qualms whatsoever pushing the limits of what is possible. I've met a lot of astronauts - I don't recall meeting a single one who didn't fully understand how dangerous the work is. It's not for everyone. Personally, I think they're nuts to take that risk for what amounts to floating around in a stinky can for a few days, punctuated by brief stints of mechanic duty.
This always struck me as a major disconnect between the brass at NASA and the engineers. The higher up the chain you went, the more concerned with safety, politics, and perception they were, and the less concerned with what I would say the true goals of the space program are - the actual exploration of space.