I think your motivations are good, but your lack of technical knowledge makes your ideas a little near-sighted.
If Mojang would rewrite JE over the course of two years, they would just use the codebase from BE and change it. There is no point in wasting hundreds of programming-hours on making something that will inherently be slower than a much-more matured project. Furthermore, the whole point of MC is that it has simple visuals: the screenshots you have attached are no doubt using shaders that are computationally expensive and need a powerful PC to use them. No magical re-write will fix that. Again, making a terrain generator similar to the first image is unrealistic as a default (maybe as a choose-able option it could work). The amount of detail and variance in the terrain would take forever to generate even a small area. If you want to look at a realistic terrain generator, look up "RealisticTerrainGenerator Biome Bundle". RTG BB is a good mix of vanilla generation that is still relatively performant (my PC is by no means weak but still struggles in the more complex biomes).
You mention how we can run MC on a $10 Pi to support your argument, but that is tremendously disingenuous—the only officially supported version is 0.6.1(?) of the Pocket Edition changed specifically for the Pi, released in 2013. You can run newer versions, but they run like hot garbage, as anyone would expect.
The issue with an infinite-Y axis isn't the performance, but the math related to it. As everyone knows, MC has dynamic lighting—the problem with an endless height then becomes how the heck does the game know which blocks are covered?
I see this response has become a giant wall of text, so I'll give you a TL;DR:
Don't let a rewrite act as a panacea for inherit limitations of Java and old hardware support. No 11-year-old is going to get their parents to buy a new $1000 PC to play a video game, much less Minecraft.