The problem is that you're using the wrong thin client technology.
The thin client shouldn't give a damn what you do to generate the desktop, just its just a physical interface to a virtual world. There is absolutely no reason this can not be standardized.
Thin Keyboards should be using Thin USB, and Thin pointing devices using Thin PointDevice connections. ThinHDMI ...
Stop buying 'solutions' from vendors and by actual solutions better known as standards.
Would you buy a PC that required a specialized video cable and monitor, keyboards with their own unique plugs that only fit into that one specific model? Would you accept it if it required you to buy a special OEM adapter to plug in your speakers?
You aren't looking at the right problem. You're still buying proprietary systems which are DESIGNED TO LOCK YOU IN TO THEIR PROFIT CENTER and then expecting it to work as if its some sort of standard.
The technology is there, but vendors don't want anything to do with it as that turns the whole thing into a race to the bottom, and kills profit margins overnight.
Someone is going to have to create a OSS (not copyleft, something everyone can use) reference implementation of the system for everyone to copy. We need another BSD sockets library kind of solution. It doesn't matter who makes proprietary crap extensions 20 years down the road once the basics are covered WELL, and that everyone does them WELL because thats the minimum expectation.