I agree with ParraEels; it's ok to be realistic about what the sporting landscape is like in England, but to outright reject any forward thinking is a tad bizarre.
If West Ham doesn't move in, why wouldn't they want extra match days at the venue, especially if it continues on as an athletics venue. The rent would be tailored to what a RL can afford, that much is obvious.
We have a new 31,000, $250mil stadium in Melbourne, for League, soccer and a union team who at this point in time are all likely to kick off their first season in the stadium well below capacity. But the potential for growth is there.
That is one noticeable difference between England and Australia, in that sports clubs over there seem to have to justify the size of their stadium. If they're not filling it out, they don't deserve it! We have massive stadiums that are less than half capacity most of the time. Over there clubs are a laughing stock if they are 2-3000 below capacity (from my experience mingling with soccer fans).
From what I understand there are a lot of interested parties: Skolars in their bid to be a SL club, 2 union teams, 2 soccer teams, and a cricket team. So no shortage of takers. I imagine though that if the Olympic committee or whoever are interested in keeping that track, and reducing capacity to 25,000 that Skolars and Leyton Orient would be right up there, even if it means moving Skolars from North London, especially because Wasps/Saracens would be foolish to move. Skolars would have nothing to lose. But yes, England's insistence on filling capacities might quash any aspirations.