There likely are many runners who would actually be much more competitive in cycling, subject to learning skills such as handling. Craig Mottram and Chris Solinsky come to mind.
Top level cyclists are those with superior power to weight ratios, particularly at certain points on their power curve (e.g. 5 min watts/kilograms). There are roles for riders who simply have off the charts watts but not w/kg (i.e. they are heavy), but ultimately those types are cannon fodder who do a job for the team leaders and then drop off.
Power to weight in cycling can be converted to running speeds relatively straight forward by comparing the energy needed (watts for time duration=>calories=>distance ran=>speed) for certain efforts. 4 w/kg for a 65kg rider comes out to something in the low 6's per mile. 5.5 w/kg is about 5 minute mile pace, and 6.5 w/kg (top top climber's threshold/30 minute power) is around 4:20/mile--also known as world class 10k pace. The numbers for elite cyclists compare well to running times across the power curve.
The difference between running and cycling is that running is all power to weight (13:00 5k is 13:00 5k), while in cycling a heavier person will be faster than a lighter person with the same power to weight because wind resistance factors in much more. A Solinsky type who can run under 27:00 for 10k but at 75kg would have potential to dominate time trials while also climbing with the best climbers--that is, a potential grand tour champion. A Bekele would be more like Nairo Quintana--great climber, but destroyed in TT.