Google Pixar etc.
The brutal reality of this is:
Software on Mainframes crashes at least as often as BSD if not more, fewer users mean fewer eyeballs have used the code and it is likely to have more bugs as a result. i.e. Websphere is likely less bug free than Apache for a similar maturity due to fewer users and fewer apps that use it.
Forget the linux slices on mainframes, they are crap, your software will crash as often, plus every now and again the mainframe will kill the whole linux slice thinking there is a problem.
Mainframes cannot offer stability so they offer 'availability', i.e. when an app takes too much processing, or disk space are whatever, it is killed and a fresh instance started. Turning a *maybe*-crash into a *real* crash.... ahh but there is another instance running in a different slice, so it's 'available' still..... well at least a new instance of it is...
IBM can't compete in the processor market, the processors in a mainframe are not better in any sense than their PC counterparts. Likewise the rest of the kit, mostly stock technology from the PC world.
IO? Again IBM do not dominate the IO world, Pixar will not switch its render farm to a mainframe, even the storage will not be switched.
Scalability? Google will not switch their search engine to a Mainframe.
Dependability?... NYSE won't switch back to mainframes.
Maturity, yes, I'll give you that, I am not an early adopter of anything, no-one should depend on something till lots and lots of user eyes have given it a clean bill of health. Likewise really old software suffers because the hardware has changed, he protocols changed, but the eyeballs don't take a second look at the new combination of new hardware plus old software. So well tested software on the older mainframe hardware it was designed, is similar to well tested software on older PC hardware it was designed for.
So if there are 500 customers of a platform, vs 5 million, then 10000 times the eyeballs. The world is not favorable to mainframes.
So what do we have? A platform with few users, few developers, no special technology, few real world reviews, but an awful lot of vague sales twaddle. I don't know what the back story is here, but this bank has made a mistake in my view.