Here’s yet another strawman. Neither I nor the letter say the UN should stop criticizing Israel altogether.
There’s no need to “imagine” the UN is biased against Israel because it is already true. As an overall body, the UN passes more resolutions criticizing Israel than any other country. This is true whether you control for population or whatever other metric you choose.
The UN Human Rights Council, to use my example again, has passed more resolutions criticizing Syria (!), Iran, and North Korea combined.