M0 is primarily to replace 8-bit MCUs, why bother with high MHz, that is just asking for trouble. In most applications a 20 MHz M0 can outperform a 60 MHz 8-bit MCU while being lower power than most 10 MHz 8-bits.
M0+ is another source of income for ARM with some improvements for the users. If new cores hit the market, it usually takes about a year until devices with the new core become available. Available = can be ordered through Digikey, not first samples in the lab.
btw. Nuvoton extended the laws of physics, the oscillator runs from -400C!.. I always though 0 Kelvin would be the limit ;-)