Re: Alternates and such. Strictly FWIW.
- Yes, alternates are a thing. Perhaps the most famous "alternate" moment was David Payne flying from the U.S. to Osaka and competing either the day he arrived or the next day in the men's 110m hurdles, when somebody else withdrew the day before competition was to begin. He went on to medal there as well as Beijing '08 and Berlin '09.
Though it is never easy, it is easier to make this kind of switch in a sprint or a jump. The track record (so to speak) of distance runners is to hold out hope for as long as possible. Des Linden in 2012 was not fit to run the marathon but still gave it a go, but had to drop out at 3 miles because of her stress fracture (
https://www.freep.com/story/sports/olympics/rio-2016/2016/08/14/rio-olympics-marathon-desiree-linden/88724312/
)
Even Gail Devers in 2004 had a bad calf. She still tried to compete but barely even made it out of the blocks. As an athlete, you hope for a miracle or hope you can power through. Especially when/if your sponsorship contacts bonus you for "competing" at Worlds or Olympics. (I have no idea of Colleen's contract or anybody else's contract state that but suspect they do.)
The biggest issue with replacing distance runners is fitness. A fourth-place finisher at nationals probably isn't training and competing on the Euro circuit and is therefore likely less fit to run on a moment's notice than, say, a hurdler or sprinter who HAS been competing.
Add to that the unique factor this year of World's being so late in the season. Mel herself said she was finishing a two-week break and they weren't going to keep training in the "hope" that a position might open up.
That said, if you make the team and compete you get a $10k bonus from USATF - every athlete, every event. That is definitely something Mel lost out on.