This is something I have thought about a lot. There are so many ways to go with it! There is a weird appeal to having 1 bike to rule them all. I think you can cover most of the spectrum from Gravel/CX/Climbing Road/Aero Road/TT with maybe 2 bikes. That is CX and an Aero Road bike.
Currently I have a Venge, which is Awesome. Tempted to get a Crux to cover the CX/Gravel stuff to replace my TCX as the Crux has quite a good aggressive geometry which would make
It a good back up road bike.
I think a CX bike like a Crux, Boone or SuperX would do for most to cover 90% of situations. So if someone put a gun to my head and said you can have one bike and NO others I would go for something like the following:
Trek Boone in Road drag, looks awesome. With 2 sets of wheels plus some different chainrings/cassettes you wouldn’t be far off a quiver killer there.
However with all that said, the aero road bike will probably always be a compromise vs an out and out TT rig. Likewise CX bike is going to be pretty aggressive for LONG gravel grinders.