A future Apple Silicon Mac Pro is not (and likely will not) be threatened by a future RTX 4090, even in the extremely unlikely event that it is actually 3x as fast as a 3090.
The reason I can say this without any doubt in my mind is simple.
An Apple Silicon Mac Pro will live or die based not on raw GPU, or even CPU performance, but on how well it succeeds as a platform.
How many of the important apps will have native Apple Silicon versions?
Can Metal make concrete progress in catching up to CUDA, Vulkan and other graphics APIs and begin to achieve real growth in adoption?
How well is Apple going to develop the capability to actually deploy the machine learning capabilities of its Macs?
Will professionals be willing to take a risk on a 2013 nMP like design with less expansion, OR will Apple dramatically expand the types of expansion options available on Apple Silicon and build a real replacement for the 2019 MP?
These and so many other questions are, IMHO what will define whether an Apple Silicon MP is "in trouble," or not.
Given all of those other hurdles, if we can make it to the point where it's actually "Do I want a PC with the 4090 or an Apple Silicon Mac Pro with an M2 Max Quad for rendering?" that in and of itself will represent a significant win in my book. As things stand right now, Apple Silicon could be, on paper, significantly faster than an RTX 4090 and you'd still have a lot of people saying "well, that's nice and all but my workflow relies on CUDA and doesn't have a Metal alternative or Metal support is still in beta and slow/unreliable"
None of this is to say raw speed isn't important just that there's so much else in play that it doesn't make sense to speculate how well a future Mac Pro (which we no very little to nothing about) compares (on paper no less) to an Nvidia GPU that... we know very little about...