That is assuming,
1. It is mostly compatible with Gems. 2. It is faster and accepted by the Core Team. 3. Most importantly, license. Both MRI and YARV, the two CRuby implmentation are all MIT. It is highly unlikely JRuby or OMR could be used as mainline.
It may be worth rembering that today's (1.9 and later) "MRI" started life as an alternative to MRI called "YARV". For a while there was a lot of talk about Rubinius as potentially being the basis of a similar replacement and becoming the "MRI" of the future. While there is less talk recently about Rubinius in that role, its absolutely not impossible that a competing core engine could be adopted as the mainline implementation of Ruby again.