The problem is, there is no chip in existence that could be used in a “Pro” to provide substantially improved performance while maintaining compatibility, the only options are
Tegra 1 overclocked - same chip as in current Switch, but massively overclocked (or more like, normally clocked). This would require a further die shrink to sustain heat and curb battery drain, and still it would not be a massive jump.
Tegra 2 - as far as I know, this chip is not suitable to a portable device with a tight power consumption and heat budget.
So short of Nvdia announcing a new Tegra chip, Nintendo has spectacularly managed to choose yet another dead-end architecture.