So what can we do to achieve the ideal distributed network? Throttle all (nodes) to the slowest... to get the best diversity? We need all (nodes), whether high or small capacity. Don't we?

Tor is a form of gravity well. If the cloud is not saturated, adding more nodes increases odds of traffic analysis. Among other things, tor's utilization falloff curve should probably not give many users and use cases much comfy feels. Tor's design doesn't provide a way to distributively utilize excess nodes safely, it cannot, so it tries to game and manage that as best it can. Such global throttle could help, but access to throughput will disappear, and it's still subject to same class of analysis. Tor's design can't really do much here while still being called tor. Look elsewhere for other designs.