Technical Reasons of stopping C# based Client (Legacy vSphere Client)

1. Does not scale per user: The API calls that the Legacy C# Client makes exert a very heavy load on vCenter Server. A few dozen connections from the client can be enough to cripple the vCenter Servers performance.
2. Does not scale for vCenter Server churn/activity/tasks: In order to maintain the Live Refresh behavior of the Legacy C# Client, it polls vCenter Server constantly. The more activity that occurs on the vCenter Server, the more processing that is required just for processing the UI for each user.
3. Does not scale for inventory: Similar to the above point, if the inventory size is large enough, the Legacy C# Client will fail to load the inventory. Some of this is scalability in the way it displays information, but it is also linked to the above, as more inventory means more churn to track, and even worse scaling.
4. Tougher to maintain as customers have to upgrade each instance of the Legacy C# Client on each machine on which it is installed for every new release of vSphere.

Read More at:-
https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2147929