> client turnover would be a leading indication of how successful the software really is Not necessarily, after the original investment and installation there

> client turnover would be a leading indication of how successful the software really is Not necessarily, after the original investment and installation there is a least one person and possible several higher ups with a vested interest in the project being a success and ending the support would make them look bad. Going through all that again and replacing your software won't happen until a new batch of managers champion the cause and will possibly happen even if your software is best in class. I think a better measure would be their willingness to adopt other products your company builds.