What would have been nice is if Juniper oversubscribed the face plate of
this platform, as most people are more likely to run out of ports than
they would the 400Gbps forwarding capacity of Trio.
You're restricted to 400G because they did fixed lane allocations to the EA chip on the PFE to each port group. Doing an MRATE setup to let you access all 480G would have increased electrical complexity, and dramatically increased the price point of the box. There are tradeoffs. The more flexibility you want, the more expensive the box is going to be.
I'm not sure they allow oversubscription on anything in the MX line anymore honestly. I could be wrong, I've been face down in a specific subset of equipment for a while, someone please correct me if I am.