I agree, Wanjiru would have been faster in 2008 and would have challenged for gold. But he still would have had to face Bekele in his prime. Injuries might have been a factor. Another one is money. Wanjiru made more running marathons than he ever could have on the track.
I'm sure that Wanjiru himself didn't move up to avoid racing Bekele. He was a fearless competitor.
But other guys who were low 27 10k guys probably did. Even if you could run 27 flat with a 55 second last lap you couldn't beat Bekele from 2003-2009.
Geb ran his first marathon in 2002, though he was still track focused until 2004. In 2005 he was undefeated on the roads. In 2006 he won Berlin and had the fastest time of the year. In 2007 he set the world record.
If Bekele wants to follow Geb's lead he needs to run a marathon this year or next year so he's ready to do it full time by 2016-7.
And even if he does eventually run under 2:04 he won't be dominant like Geb was. He will HAVE to run that fast, just to be competitive with Mutai, Makau, Mosop, Kipsang, Kirui, etc, etc, etc.