.>> Abrupt changes that require customers or practitioners to write off existing investment and re-invest with the same vendor, are almost unthinkable.

I agree. But that wasn't the case with VFP. It wasn't even close to being abrupt.
Anyone whose head wasn't in the sand could see that coming years before it did.
As early as the late nineties (not the gay ones) my prospects' IT people were voicing doubts about its viability and after the millenium it was an outright negative if you proposed it as a development tool.
I lost several large bids because of it years before they axed it,


A better example is Silverlight. That was "unthinkable" in many ways but mainly in its sheer stupidity.
I was attending MS Workshops about it while they were killing it!
You just don't do things like that to your clients' investments in your products.