I'd wager
even if IBM sold x-series to lenovo or whoever they'd still use those systems for years to come. They obviously know the platform well and have it widely deployed, integrated etc. They would have wonderful pricing power etc.. there's just no reason for them to use anything else.
Another option would be keep a specialist part of the X-series team in house (similar to Dell DCS or something) for hyperscale configurations and develop that for use in their service provider cloud stuff.