My dog had double TPLO a few years ago, it changed his life. He was 3 years old and almost immobilized, low quality of life, and the surgery brought him back to normal after recovery. Our vet recommended doing them one at a time to help with healing. His play buddy got the same surgery almost exactly the same time but both legs at once, and had some pretty severe complications. Anecdotal I know, but it did play out just as our vet described.
Procedure cost around $10k all in. It's expensive, but worth it for the dog if you can.
Yes, that's the short of it. Each pixel needs its own wires, readout, and processing chain, and resources are limited on the spacecraft. The cryostat (instrument that keeps the pixels cold) only has so much cooling capacity and all the wires add thermal load.
Future missions are planned with more pixels (Take a look at the EASA Athena mission and its X-IFU instrument), and to reach that goal they are using multiplexing methods to allow more pixels to run on fewer wires.