SpaceX Dragon will be able to pickup the slack within months.
I doubt it. You can’t put dragon engines on an atlas or delta. The payloads for those two vehicles are designed for those vehicles only. Further, you can not put an atlas or delta payload on a dragon for dozens of reason (weight, CG, com links, size, dimensions, vibration, etc.)
If the Russians deny access to the ISS, there will be nowhere for the Dragon module to go. If Obama and Putin continue on this path, will the motivation and funding for replacement delivery systems to the ISS wither away?