Lagat's performance in London last week showed that he has fallen off quite a bit. In a time trial type race, he ran 12:53, narrowly getting outkicked by Farah in 2011. In 2016, he's 21 seconds slower and losing to Farah by 15 seconds. To me, that shows natural regression with age. Doped or not doped, I don't know, but his performances this year are nothing out of the ordinary.
Blame sit and kick for someone like Lagat being competitive in the US after age 40. Even Meb; shouldn't be able to place top 3 in the marathon with a 2:12. US distance runners are just not that good. That is reality.