How do you know that? A ton of strikers in MMA have developed decent wrestling defense for MMA (Cro Cop, Jose Aldo, Tarec Saffedine, recently Anderson Silva) so why do you assume it's impossible for an elite boxer to do the same? There are literally no examples to back up that claim since no elite boxers has ever seriously trained MMA (Mercer and Toney at this point are NOT elite boxers nor did they seriously train MMA).
Koscheck, Maynard, Jon Jones and Edgar are garbage strikers in the grand scheme of things. The guys you mentioned have not reached an elite level of striking by any stretch of the imagination. They're college (not even close to world class) level wrestlers with mediocre to bad striking who are out striking other MMA guys with worse striking. That's just not a good measure of elite striking skills.
Small MT strikers like Buakaw would put Jon Jones on his ass in a kickboxing match and an old 5 foot 7 Jose Luis Castillo would KO him through the ropes in a boxing match. The level of striking in MMA is pure ass and really only Anderson Silva has reached a respectable level. Even other elite strikers like Shogun look like trash compared to boxers (nikka's punching technique is
and his defense isn't good either). The GRAPPLING can be elite (Werdum, Magalaes, Shalorus, Maia, Askren, Jacare, Cormier, Rousey, Roger Gracie, King Mo etc.) but even then most MMA fighters are still just above average on the world scale. It's the mixture of the arts together that makes MMA fighters elite.
There's a hell of a lot of technique! Guys just aren't as good at what they do as boxers. It's not even about the disciplines themeselves, it's just that boxing has a way better, way bigger talent pool and over a hundred years of refinement and trial and error. MMA will get better when athletes who are good by most standards (like Jon Jones), not just MMA standards, start training the sport. That won't happen though unless the UFC starts paying these guys and BJJ becomes a realistic option for inner city youths.