By that logic, Apple should have been sunk in 1992, when it discontinued the still-highly-popular Apple ][ series in favor of the Mac; or in 1994, when the venerable Motorola 680xx processor was abandoned in favor of the PowerPC, orphaning users of software whose developers supported only the newer architecture from then on; or in 2002, when the last OS 9-bootable Macs rolled off the production line, leaving behind a large and often angry population of OS 9 users (*cough*grelber*cough*); or in 2006, when the switch to Intel processors put the Classic environment out to pasture; or with the introduction of Snow Leopard itself, which jettisoned the remaining PPC Mac owners in one fell swoop.
What makes you think the 1 in 5 Mac users still on Snow Leopard constitute a unique class?
I think that unlike any of the events you've enumerated, Apple's apparent treatment of Snowy users rises to the level of abandonment.
(As far as I'm concerned, this discussion is over, 'cause ain't nobody changin' nobody's mind; we can revisit the moment in 10 or 20 years when its impact, if any, has been quantified.)
