This obviously has no relevance for starlink which does not have local datacenters for cdn purposes. All that bandwidth is going through the satellites right before it reaches the user.
Imagine they put 10TB of flash memory on the satellites and run virtual machines for the big CDN companies (cloudflare, Google, Netflix etc).
I reckon that 10TB is still big enough to service a good little chunk of internet traffic.
You have to share that 10 TB with everything on that satellite's orbit.
Most ISPs have CND appliances in their racks to save on uplink bandwidth. And from a satellite perspective the uplink (in this scenario: the downlink from the satellite to the gateway) definitely is the expensive bottleneck.
You want to avoid congestion and every bit of caching could be helpful.
Then it comes down to the mass and power budget (and the reliability of flash drives in space) - but that doesn't seem too terrible.