One argument in favor of a Microsoft-centric solution is that decent Microsoft resources that can support and enhance the application are more abundant and easier to find. For a smaller client lacking in-house resources this flexibility can be a huge difference maker alone. Given that, its probably still in your best interest to learn to develop on PHP/MySQL platform. Especially since then you can build more cost-effective solutions for your clients AND be the one they know to turn to in the future.

Not only is it possible, Koios Works has released multiple games all developed in C#. Their latest game, Panzer Command: Operation Winter Storm just came out in June.