>Or is this side of IBM stable enough for buyers to have confidence that it's still going to be there for service, support and spares?
Yes. This business is stable and profitable regardless of whether IBM is solvent in the near future, it will be enthusiastically snatched up from Chapter 11 by any of more than a dozen large tech companies, where that is HP, Dell, Fujitsu or Oracle.
The people with big iron will always eventually need replacement big iron, so long as it's cheaper than refactoring their legacy codebase. Collectively almost a trillion dollars worth of technical debt is invested in Power and s360/370/390/zArchitecture.