And in the real world...
IBM never had a monopoly on mainframes. What they had is a monopoly on IBM architecture mainframes (although RCA once produced a semi-compatible called the Spectra which English Electric Computers and, later, ICL sold under license as System 4 - it was a financial disaster for RCA). However, Honeywell and a number of others had alternative, but incompatible mainframes.
What IBM had was a de-facto control of a large part of the market due to their dominance and the immense difficulty of customers migrating to alternatives. That's pretty well the position if you are an Oracle shop, using Oracle Databases and Oracle-owned middleware. Once Oracle start eliminating the hardware competition by simply not developing, supporting or releasing
I also had personal experience of Oracle putting pressure on continuing to use SPARC hardware rather than x64. On this (very sensitive and critical application) the sales people started hinting to business managers that they could only provide the very best support on Oracle hardware. The irony was that Oracle had mightily screwed up on a related application on SPARC hardware before they'd bought the company. They actually had the cheek to state the reason for this was they were separate companies then and it was an issue of miscommunication that would not happen now they were one. When challenged directly that this meant they were saying they'd give poorer support on Oracle with x64 and Linux versus SPARC and Solaris, they backed down - sort of. They were still trying to sow seeds of doubt. The hardware cost differences are huge - an M-series solution versus a comparable x64 LINUX one is around 5:1 (and getting wider). T series is something like 3:1 on throughput (and many apps simply don't run fast enough on T series).
Now Oracle aren't alone in this - salesman have to eat, and they'll use what tactics they can. But to be fair to IBM they are now much less likely to throw FUD around than Oracle are now when it comes to compatibility issues.
Needless to say the details of this have to remain anonymous, but be of no misunderstanding, sales teams will use all the pressure they can, and hinting at instability of Oracle on non-Oracle hardware is now one of them. They will also do this to senior business managers.
I could name a few other companies that can be ruthless in this way - once customers are tied into a software solution which is disproportionately expensive to migrate, this is bound to happen. What we have now with Oracle is a number of previously independent companies which were once largely hardware-neutral (including Oracle itself) being turned into a vertically-integrated organisation which will increasingly squeezed out competitions.