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What if dotNET fails? Or, what if dotNET does not become stable for a few years? During the time a devloper was learning dotNET, they could have been learning VFP/Web Connection. VFP/Web Connection works now. How is that zero opportunity costs?
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There is no crystal ball here. And FWIW, whether dotNET fails or not has no impact on VFP. Further, investing in Web Connection is not what I would consider to be investing in VFP per se, but rather a specific product. Still, I will go ahead and concede that point to you because it is irrelevant.

Whether I spend zero dollars or 10,000 dollars, my knowledge about VFP or what I can do with VFP is the same. If anything, there is a dis-incentive to make further investment because of the lack of marginal utility.

That is how it is zero opp cost.