300 MIPS is sufficient for today's spacecraft only because they have to design said spacecraft around the RAD750's limitations. They'd love to be able to move on to something faster, and in fact NASA has a program under way to do just that. The RAD750 is getting very old, and it's a real limitation.
To give an example... iirc, the JWST is bottlenecked not by its various observational instruments, but by how fast its radios can send collected data to earth. If they had more local compute, they could use better compression algorithms to get more out of the available bandwidth.
Another example... the Ingenuity helicopter drone deployed by the recent Perseverance Mars rover had to fly autonomously, meaning they needed profoundly better onboard compute, weight, and power efficiency than a RAD750. There was literally no rad hardened system which could meet the program's requirements, so they put in an off the shelf Qualcomm Snapdragon 801 smartphone SoC, and lived with the risks. Only possible because the drone was viewed as a sort of bonus mission, and added little or no risk to the main rover mission, but it would've been a total nonstarter with RAD750 compute performance.
The reason things move so slowly comes down to money. It's expensive to design and qualify rad-hard CPUs, even when starting from an existing non-hardened design as the RAD750 did. Same applies to systems. Extra development expenses relative to commercial combined with ultra-low sales volume means there's no economical way to move faster.
