If you are going to carry multiple product lines, then why would their features converge? I see no reason to get into the closed vs open debate - this is Apple we're talking about.
But why would they turn the Mac Book pro into a MacBook Air with higher specs? Why not get rid of all the branding distinctions and let the consumer spec out the machine, with a few default configurations for those who don't want to think about specs?
They didn't. You can still get an old-style macbook pro with updated internals. They're offering a new option that looks a lot like a MBP-Air. Doing essentially what you seem to suggest: giving people a choice of a new thing and not taking any old things away.
And if/when most pros demonstrate that they don't care about the modicum of hardware-accessibility in the old-style pro design, Apple will simply be dropping the product that its customers do not prefer. As it always does. As it did with the 17" MBP. A beast that increasingly few Apple customers had any interest in.
You really think Apple will be selling old-style MBPs indefinitely? It seems like the product line is going through a transition, and the old-style MBPs will be the end of the line.
After all -- despite the mini, nano and shuffle -- the iPod classic stubbornly remains. Even after all these years. Not to mention the 13" MBP which seems (without the benefit of sales data) to be fairly pointless.
Apple won't hesitate to risk cannibalizing itself with new products. But they don't have any history of forcing the issue, if the old products still sell.
There are completely different chipsets, ports, batteries etc in a MacBook Air versus a MacBook Pro. Most of which have dependencies on each other. So a user can't just spec out a machine with the exception of very specific components.
Hence why they are branded differently.
The MB pro is almost as thin, has a SSD, integrated battery, an no disk drive and a better screen.
They killed the old Mack Book Line.
That is one of Apple's greatest strengths, and having that strength absolutely allows them to offer configurability.
It does NOT mean a person who wants the ability to upgrade their RAM/Hard Disk from a third party.