That's not an insult to say that you start to decline a little before your 30.
Bekele went for Marathon when he couldn't do anymore what he was able to do on the tracks. He had lost his cutting edge speed. Marathon is a different beast so you can perform very well even after 30, but still, being in your physical prime always happen before 30, not after.
There is no doubt Bekele declined well before going to marathon and was not able run the 10k like he used to.
I understand that the record was slower at that time and that the runners have progressed since. But I would have loved to see a prime Bekele unleashing his full potential on the marathon.