I will repeat my argument.
If you don't want to use GPL, for whatever reason, more power to you. I respect your right to publish your code under the license you choose. I hope you respect my right to publish my code under GPL.
If I don't accept your license, I will not use your code. If you don't accept GPL, nobody forces you, or your clients, to use my code. As I said I am sure there are many commercial options.
About your example. Assuming that you used my GPL software, you would be just one of my clients. Thus if you gave me back any changes you made, but I could not redistribute them, it would be not enough a compensation. I want to be able to redistribute the code so that I get more contributions from others.
By the way, if you client combined your code (you give your clients the source as you said) with GPL code, it would not be a problem at all. Since you forbid them to redistribute your code anyway, GPL explicitly allows them to change GPL code, without giving anything back.