libsoup 3 and libsoup 2
I'm aware of #218 and #202 (closed), but I did not find a discussion why to avoid compilation of the code which uses both libsoup-2.4 and libsoup3. I do not mean the application itself doing that, but when the application uses libsoup2 and one (or more) of its libraries use libsoup3. That's easily achievable, in either way (app being ported, libs not, or vice versa, or some libs ported and some not).
In case of the evolution-data-server the libedataserver exposes libsoup in the public API and it uses libgdata and libgweather (to name the two as an example), where each of these three can use either libsoup version. Being able to use both versions in runtime is kind of essential, for such core library the libsoup is. All these libraries are used by evolution, where evo itself uses certain libsoup version as well.
Look on gtk3, there never happened a complete transition from gtk2 to gtk3 for all applications.
You have a lengthy list of the dependencies in the #218, but there are some missing, some 3rd-party applications, which have their own API stability promises. One being syncevolution, which is not under the GNOME umbrella. There might be a lot of others.
A nature question comes: do you really expect that all the projects mentioned in #218, and those you are not currently aware of, will switch to libsoup3 at the same time? Most of the projects are heavily understaffed, or they lost their main developer some time ago. The "part time" maintainers/developers might not have enough time to do the porting. Not talking that the dependencies spread the need a lot, because porting one library means porting tens of others. Maybe not tens, but you know what I mean. The infamous dependency hell. Though the coordination with all the projects is probably the most problematic part of your plan.