> My responsibility is to the client.
As it is for all of us.
However, what is best for the client often exists in the eye of the consultant.

This has been my approach to my VFP clients:
About 15 years ago I gave them all a choice - let us change the backend to SQL Server or get someone else.
They all went along.
We did it incrementally. As functions changed, we did the conversions. All new tables are SQL Server.
There are still a few small .DBF's but all the large tables have been converted.
The last large one bit the dust just last month.
All new work uses SQL Server. I haven't created a new DBF in over 10 years.
If I have to defend why I insisted that they switch to SQL Server, then you and I have a different definition of our responsibilities to our clients.

Regarding using .NET.
All new applications are written in .NET.
I wouldn't convert a VFP application to .NET for the sake of converting it, but with each change to a VFP application we ask the question "Can we do this change more efficiently if we convert the app to .NET?" and proceed accordingly.
We recently converted a VFP6 application to VFP9 because that platform gave us what we wanted at a fraction of the cost of converting it to .NET.

Finally- I think I that one responsibility to our clients is to stay healthy as a business.
I think we do that best by maintaining a solid technical arsenal that we can offer existing and potential clients.