It could be the case that that is true.
If I were a business owner, that is not a bet I'd make.
1. z-series experts are rare, expensive and typically old. Old does not mean unemployable, but it does mean a limited future career. Who's going to replace the current crop of experts? Internal education is a possibility that could work, but any way you cut the knot, it'll be more difficult than going with x86 and linux.
2. z-series hardware sucks. They are low volume parts who's main value is their backwards compatibility. If you think the hardware performance is acceptable today, will it be so in 10 years? Computer performance improvements have certainly slowed down, but peripheral vendors like nVidia continue to make solid progress. Even if IBM gets around to supporting them, you'll always be a 2nd class citizen compared to the PC platform: drivers, bus availability, etc. will be half baked or late.
3. z-series is off the main stream of computing. Huge numbers of talented software developers are building great open source software for linux (and other OS's) on x86. I don't know if administration on z-series can be done better or less expensively than on x86 today. I feel confident in saying that there will be no contest in 10-15 years. The pace of innovation is enormous, and IBM is not a company that can credibly make a commitment to bring that innovation to the mainframe.
4. Yes, mainframes can run linux and do various sorts of emulation. I don't think that that is a cost effective. I'm not alone in that assessment: if running mainframes was a cost effective way to run linux, I'd expect that to be the way big internet businesses run their internal data centers. They don't.
5. You'd have to deal with IBM sales. At least with x86, you have multiple sources for parts, and if you're big enough you can do contract manufacturing.
6. The markups for EC2 (especially the network transit) is really high. There is a continuum from Lambda and App Engine, through, EC2 to dedicated hardware rental like OVH and finally running your own. That is way more options than you have running on a mainframe: certainly way more vendors.
P.S. Huge fan of your writing.