I'll second the general consensus here so far... I routinely re-pitch yeast in this way, since I've rarely had time to brew lately and no LHBS.
In my experience the "doom-and-gloom" has been accurate. I can't say to have much or any experience outside of a few strains, but broadly speaking I think viability of relatively small amounts of slurry stored under beer drop into the 80s relatively quickly (a few weeks) but then decline more slowly. I've seen 55% in a 19-month old slurry of 1272, though methylene blue staining is known to be inconsistent below 75%-ish (and even in general) so take that for what it's worth.
Long story short, take a couple tablespoons of packed slurry (30 mL), that would be about equivalent to a fresh smack pack or vial or whatnot when harvested, so even if viability is down to 50% you're in the ballpark. Give them oxygen and nutrients and yeast will reproduce until they run out of substrate, and a 1 L starter at 8°P will net something like 100-150 billion fresh cells in my (again, strain-limited) experience. (And for my brewing practices would be slightly underpitching in a high-gravity beer, but take that for what it's worth.)