I agree that the TC4 is a great car that has stood the test of time. I loved my HPI Pro4, but parts that werent interchangable with the TCX/TCXX were getting very hard to find. Even on Ebay.
Both those cars could go toe to toe with the newest of cars with only minor changes.
TC4: amodern material chassis and towers. Metric hardware (only because its the only standard hardware car left around). And gear diffs.
Pro4: Lower shock towers and short shocks, updated lipo chassis similar to exotek, and gear diffs.
The US market may small, but I promise you, its Xrays bread and butter right now. Im sure thy sell plenty of kits elswere in the world, but without the US, they wouldnt be anywhere near as large as they are now. There is just too much competition outside of the US. Also the US is almost entirely a spec market. It makes it much easier to target.
As far as money into R&D, with cheap 3D printing available now, creating parts for testing should be much less expensive and time consuming than before.