This is true, but I'd guess that having a nice Mac Pro isn't going to gain them market share. The market they're losing is probably the low end (where cost difference is higher).
Sure, the volume is definitely in the low end. But the cost difference has always been there, and they used to have higher market share.
They're losing 2 things:
1. Exclusivity of really good hardware and software. They're still among the best in hardware, but a nice 'ultrabook' (like a Surface Book, XPS 13, or X1 Carbon) running Windows 10 is comparable to a Mac in ways that an Inspiron 6400 running Windows XP or Vista just wasn't comparable to a 2007 Macbook Pro running OS X 10.5.
2. The dedication of the high end. The developers on this site are a tiny market, for certain. But we're the people our families and friends go to for computer questions (for better or for worse). The artists and especially the musicians and videographers which the Mac Pro used to attract are more trendsetters. If enough high schoolers watch their favorite Youtubers editing on their Mac Pros, and ask their uncle to look up from reading HN on his Macbook Pro to recommend a laptop, that effect can have a powerful impact on the low end.
Lexus offers the LFA because it attracts enthusiasts and media to enhance the brand and to ensure the pleb models have a higher perceived value.
Apple offering an extremely obsolete Mac Pro filters down from a brand standpoint, it hurts them by turning former ambassadors against them (such as myself), and by losing third party support for professional software when the customer base starts asking for alternatives because the hardware is too obsolete.
They're losing 2 things:
1. Exclusivity of really good hardware and software. They're still among the best in hardware, but a nice 'ultrabook' (like a Surface Book, XPS 13, or X1 Carbon) running Windows 10 is comparable to a Mac in ways that an Inspiron 6400 running Windows XP or Vista just wasn't comparable to a 2007 Macbook Pro running OS X 10.5.
2. The dedication of the high end. The developers on this site are a tiny market, for certain. But we're the people our families and friends go to for computer questions (for better or for worse). The artists and especially the musicians and videographers which the Mac Pro used to attract are more trendsetters. If enough high schoolers watch their favorite Youtubers editing on their Mac Pros, and ask their uncle to look up from reading HN on his Macbook Pro to recommend a laptop, that effect can have a powerful impact on the low end.