Personally I don't think Apple's decisions about previous OS version support are much—if at all—influenced by holdout use(rs). As the Computerworld article I linked to opined, those decisions seem to have more to do with how many versions have succeeded the one at issue than with how many users are still using it*. But as new versions appear with increased frequency, the pressure on the about-to-be-dropped oldest one may be more obvious than before, particularly if there's something 'special' about it like Rosetta support in Snowy. Also, support for the oldest supported version may be partial compared to that for more current ones, as evidenced by Apple's response to past malware outbreaks.
And as to 'disenfranchising' users of older tech, I'd like to add that for example PPC Mac users now are dealing with formally obsoleted hardware, the last such Mac (running Leopard at best) having been made about 8 years ago. Their usability continues to diminish, especially regarding the Internet. In that respect, those Macs fully match previous generations. The oldest Intel Macs are following closely behind...
*) Note that the article includes an OS version usage graph for Nov. 2013, uncluttered by non-Mac OSes. You'd need similar usage stats for each of those older versions to document holdout numbers etc. at the time support was dropped, but history seems to support this general idea.
