Okay, rushing in here where angels fear to tread and fully expecting to be corrected by the real luthiers on this forum, I will address your comments from the perspective of a non-profession, non-orchestral bassist. Which means that much of what I say here applies to the world inhabited by @damonsmith or, apparently, yourself.
1. f makes for a much longer weaker neck (every half step prolongs the neck by an inch).
Necks can be profiled to add the needed strength. Even with D and Eb necks now, there are different profiles (thicknesses). I think a greater issue would be the inevitable making the mensure longer.
2. The heel intrudes on the upper bout (nothing comes free), placing it to far down means a smaller plate area/air volume.
This, the intrusion, I admit I am having a hard time visualizing. Do the heels on the D and Eb necks not intrude? To counter this, one might shave the heel further down, which would weaken the heel. Or one might deepen the mortise to bury the heel further in, which would shorten the mensure. But 3/4 bass mensures already range from 42.5 to 40 inches, so where is the problem? And if one can deepen the mortise without adding appreciably to the size of the neck block, how much will the plate area/air volume be affected?
An even more radical (and much uglier) approach would be to dispense with the visible heel entirely, so that what the eye sees resembles a flat surface. The entire "heel" disappears into the mortise; what we see is what we would see if we looked at a P bass with its neck removed. And I'll bet that would give you your F neck.
3. The heel also acts as a reference point for the left hand, putting it to far away from the nut could make you a little "lost".
But aren't you already pretty far away from the nut even on a D neck? My understanding is that the reference point should be the note at the heel, not distance from the nut.
3. eb is enough for achieving the two main goals: pivot up to the first octave, reaching comfortably all notes on a 3 octave fingerboard.
Point conceded, for bassists who inhabit yours and Damon's world. The opportunities for this sort of variation from the norm are very limited. But folkies like me have more options; if we are curious about something and can find it, go for it! Even in the upper echelons of bass making experiments are departing from the "traditional" bass formula: changes in bracing, in body shape, etc. Some of the luthiers are pushing the envelope.
4. if you want to switch seamless between say 2 basses, there's not a lot f to choose among out there.
You got me here, but if F basses were more accepted there would be more of them. Full disclosure: my two best basses are a ply and a hybrid both out of the same factory. Both have Eb necks. I have small hands and short fingers. If there were a F bass available I would have leapt on it. A G bass would have been even better.
5. You could loan your bass to Gary Karr if he should turn up in your hometown for a concert.