The main thing buying with a US-issued CC gets you is a refund if the airlines do not deliver the services promised. And that's great, but it is not a replacement ticket.

Also consider that S7 (and the other Russian airlines) have been seen as financially unstable and at risk of failure for a little while now. Were that to happen there is the chance that AB would not honor the ticket as it would not be paid for such, but that's mostly unlikely. Still worth considering.