Thanks for the post.
The GL caps are just one dimension - each major OS version requires tweaks to account for subtle differences in behaviour between version; sometimes the tweaks are different for Intel/ATI/nVidia GPUs. By limiting the number of old versions we support and test we free up engineers to do things like the IOAcelerated framebuffer driver for Lion and Mountain Lion included in VMware Fusion 6.
Of course there are other considerations including build tools, as you mentioned, and non-graphics framework features that become available in newer versions of Mac OS X.
The data you cite doesn't mesh with the usage information for VMware Fusion. Of course no data-set is perfect but the change was made with an eye to the impact on our customers. Of course, older versions of VMware Fusion continue to be available from the MyVMware portal (the feature is still there, the version drop-down box moved during the redesign.)
-Simon