I'll kill both birds with one stone here... Apple gives product hints to their big buyers (including pro buyers.)
You've mentioned this before, but I've never actually seen evidence of this or had any other source validate these claims. So such direct information such as this must be rare indeed. And if this only occurs for an EOL product replaced with a new product, even more so (since such an event isn't that frequent vs. typical product release cycles).
And that brings us to the real issue, which is that
large/big buyers are not the MP's target sales audience, and ultimately, not where the MP's income is primarily derived. Most of the creative work is done by independents and SMB's (even the "big" guys don't do all of the work, they subcontract it out - just look at the credits in any effects-heavy movie = litany of small companies credited for their part, such as one for smoke, another for water, ... and so on sorts of division of labor).
So leaving their primary purchasers in the dark is bad business IMO, as even though they're only buying a machine or two at a time, there are far more of them = majority of sales are from small quantity purchases by independent/SMB's.
It doesn't make any logical sense. Paranoia or lack of caring would explain it however, and there is some evidence to support that idea in a growing number of MP owners. They're concerned, and rightly so. If Apple has no intention of abandoning these people, all they need to do, is make a public statement in order to reassure them.
Rather simple, but it's a step they're not bothering with, which further increases the worry in potential buyers. Such an issue is at least as dangerous as the Osbourne Effect IMO, as the lack of information feeds the fear, which can push buyers to seek alternative solutions.
Regarding OSX it depends how much costly software would need to be switched. Cost of license swaps (available from most software companies if you ask, but cost varies) or competing software solutions as well as familiarity can make people want to remain on a particular platform.
My own primary issue with recent OSX versions has revolved around driver issues (speaking of displays, seeing 10 bit capable drivers under Lion would be nice) and bugs. It's like they're just letting it coast on the current brand popularity.
The licensing costs will vary from nothing to zero credit, and have to completely re-purchase another license for the intended platform per application suite.
But from a long-term analysis, if switching to a new platform stabilizes their work environment and expenses, it's the better way to go. Even more so if the productivity is increased (i.e. getting access to new features earlier as the software vendor will provide them in their biggest sellers first, then work their way down, assuming it's possible to add those features under a particular platform).
Given your own statements, even graphics card drivers could be viewed this way (10 bit instead of 8 bit) combined with faster bug fixes (bugs will happen, so the speed of correction is important).