A further part of the problem in my case is that the high-ranking sites that comprise the better part of my site's link profile are themselves slipping down the SERPs.

I don't see much cause for hope in my own sector, as the current page 1 (spreading down page 2) results for most of the main key terms make very little sense to me: it looks like a black-hat convention with a couple of "how on earth did that get there?" sites thrown in (not major players, not linked to, not well-designed, not much content, not large sites). This has persisted for several months, although I'm confident that it isn't permanent.

I agree that "waiting for Penguin" is futile, but building links isn't much use if you don't have good fresh content and an ongoing commitment to refreshing it. The site itself must come before the links. Keep writing and renewing (and reviewing) content, and keep your site looking fresh.

I'm still getting a reasonable amount of business from organics, but I'm no longer sure that "building up traffic generating links" is using my time as effectively as building up a network of contacts on the ground (which also make me less dependent on Google). Marketing is a question of making sure everyone knows about you and your product, and my current view is that organic search results will never regain the value they had four or five years ago for spreading that awareness. Whatever the Penguin update does, it won't make the results look like they did in 2011.

I had quite a few differences of opinion with fathom regarding disavow and the potential benefits of using/not using that particular tool to clean a link profile, or at least put a spiffy little shine on that link profile. I have since kind of come around to the viewpoint that disavow is pretty pointless. On multiple sites I have taken minimal disavows, expanded them greatly, updated them regularly and seen virtually no change to the traffic/analytics of those sites.

The rational course at this point is to realize that search engines in most cases will correctly identify and minimize the impact of un-natural links. New, earned links that bring traffic will, as goodroi points out, provide the search engines with a fundamental reason for higher rankings.

Wilburforce's noting of high ranking linking sites slipping in the SERPs should also be considered. When the strength of your inbound links slip, it's fairly easy to conclude those slips will impact your SERP positions.

It's not going to get any easier, I think that is fairly obvious.