EPO aside, money has shifted to the roads.
I remember some LRC article a couple of years ago showing the prize money for the top places in a Diamond League 5,000 m and I was shocked at how small it was. Even for the winner!
Obviously, well-established runners like Farah, Kszczot, Kendricks, Muir, Hassan, etc., have nothing to worry about because they have great sponsorship deals but if I was a young runner having to bust a nut to try and consistently place top-3 in the Diamond League against enormous competition or hopping from road race to road race in the US, Asia or Europe, where you can earn a living racing two 62-64 half-marathons a month, I might opt for the second option if I was talented, but not 60 HM or 13:00 5K-talented.