My limited understanding is that most mainframe customers are locked-in, e.g. they have legacy COBOL code running their ledger system and the expense to switch off of it is simply prohibitive. That plus the fact that the system is reliable, low-maintenance, etc. preserves the status quo, despite the fact that if you were to write the same applications today, you'd choose a newer platform because it would be more cost-effective.
As such, IBM has historically offered z/Linux and co-processors only to hold onto data and processing that was being pulled off of the mainframe because it was too expensive/too onerous to do on the mainframe using z/OS and the like. So customers, unable to completely shut down their mainframe, could kind of make the best of a bad situation and at least get some cheaper Linux cycles out of their expensive iron.
If the above is true (and correct me if it's not) what is the appeal of a Linux-only mainframe? Or is it only interesting if it is radically cheaper than a z13?