With the carpet peeled back, you can see the problem:
The platform was actually fine for storage, but it was not ideal for crawling around on if you weigh the best part of 200 lbs. I would probably not have forked out for brand new plywood to fix this issue, but since I have so much on hand, why not? I started with the crappiest bits of plywood since none of it will be seen.
Lower platform done: