That's not the only reason.Companies don't keep a product around because they feel obliged. They do so, because they feel it can make a profit. When that that product no longer generates sufficient revenue, they stop making it. Apple did this with the 17" MBP.
First, corporate contracts. Contracts Apple has to supply particular units to particular companies and contracts Apple has with manufacturers to assemble those units.
Those contracts are generally confidential and can be the "missing puzzle piece" when one wonders why a product that appears to be languishing, still exists.
Second, pricing strategy. Many are familiar with the concept of "loss leader", but the opposite concept also exists... having a higher priced model that causes people to purchase a lower priced model... there are a variety of reasons for wanting to do that.
I suspect that Apple kept the Mac Pro unchanged for so long so that they could move those sales to the Macbook Pro. (Not that the MBP is functionally equivalent to the Mac Pro) Then, once having caused that segment of their customers to "dumb down" their workflows to fit within the capabilities of the MBP, they would discontinue the Mac Pro with little pushback.
They realized by not providing a computer that meets the needs to certain demographics they were leaving money on the table and they'd be better suited to trying to retain those dedicated customers. Whether that happens, is another question but the pow-wow they held was a step in the right direction.
I think that Apple's turnaround in their decision has more to do with the down-product sales. The new MBP is most likely not selling as well as they expected.