What makes Fedora strong compared to Microsoft? Microsoft is the dominant player, Microsoft can employ many more designers— and out spend Fedora in every way, Microsoft can set a more consistent vision for the developers of the software they ship. Microsoft can hire almost any talent they like. These are all formidable strengths and they have many others.
What can't Microsoft do? Their business model is fundamentally incompatible with giving users an operating system which they— or their designees— can easily and legally change. Microsoft can't deliver a self-contained self-hosting system that with a few commands can patch and recompile anything it runs. It's incompatible for giving users access to the complete source code for everything they run, thus lowering the learning curve and turning out an endless stream of new developers.
Linux distributions could start engaging in top to bottom signing of userspace binaries— but in doing so it would be weakening the strengths of the open world and playing into pattern of top down control which they are fundamentally better at. Even if there are magic buttons and procedures you can add to regain these freedoms, and even if you ignore that one of the most important software freedom's is the freedom to share the results of your modifications and have other people able to use them easily... the additional friction of having to disable that 'security' diminishes our communities advantages.
There absolutely are applications for locked down machines where doing so provides a distinct advantage to the owner of the machine. But these applications are not typical for Fedora. I think it would be arguably in the GNU/Linux communities long term to leave those markets to special case distributions— or even to Microsoft.
I strongly support improved security— but there are a great many things which can be done to provide a more material improvement than codesigning without the compromises. The immutable boot only lets the horse be put back in the barn without reimaging the system— but by then it may be too late. At the point of initial compromise the users data may have been copied and erased, their bank accounts emptied, bitcoins stolen, whathave you.
If all you really care about is making sure that botnets don't persist then perhaps secureboot plus a sufficient amount of signed userspace is enough. But it's pretty weak from the perspective of actually securing the user.
We're hardly making use of SELinux, for example— Why are programs like Pidgin, Evince, and Firefox able to access my home directory except via a carefully audited privileged separated filechooser app? Why aren't they running in a sandbox? Why have we not built accelerated versions of tools like valgrind which are able to provide even stronger sandboxing than SELinux around the most vulnerable code? I could fill pages of things which would provide more security improvement for users than code signing without diminishing our fundamental advantage vs the closed software world. If it's security we're trying to get— why would we first do the one thing that makes us more like Microsoft?