On the other hand, you can get rack-mount x86 server with several TB of RAM for fraction of price of mainframe which is exactly the hardware for new applications that would otherwise be best served by mainframe.
I expect the largest one of these is still a fraction the size of the largest mainframe, at least for processor power (e.g. 224 cores[0] compared to 1700[1]). Maximum memory, though, is 32TB, which isn't exactly huge compared to 12TB [0] or 24TB [2].
[0] https://www.supermicro.com/products/system/7U/7089/SYS-7089P...
[1] https://en.wikipedia.org/wiki/IBM_zEnterprise_System#z14 (Not saying they're necessarily equivalent in performance, but they might be, as you point out)
[2] https://www.supermicro.com/products/system/7U/7088/SYS-7088B...
That's my general impression, as well, but I have little enough (or inadequately broad) direct experience for that impression to be a strong one.
I've also seen, second-hand (i.e. benchmarks, so not quite real-world) that cache and memory latencies (including inter-CPU/NUMA) can significantly affect OLTP performance.
If that weren't true in practice, then the sheer bandwidth one can deliver from enough SSDs over PCIe rivaling a CPU's memory bandwidth would mean main memory size is much less relevant.
And for the elasticity part, mainframes are where the whole virtualization and hypervisors started. Although, or maybe because, purchasing and operating an mainframe is significant investment in terms of both capex and opex (it is somewhat telling that IBM's specification sheets for mainframes read somewhat like flyers for new cars, about half of the thing is about available financing methods :)).