That's a premise so inane that it goes right into meaninglessness. Everything is "conceivable", especially "something that works as well as what I'm using in every way, but doesn't have problem X". It does nothing to conceive that, though.
As to what Facebook's building, meh. Anyone can try to build a better-in-every-way-for-every-application VCS, but look to TFA for a list of just some of the failed attempts to produce better version control mousetraps. It's more likely that they'll produce something that will be handy for niche uses than something that will be a clear win over git for everyone else.
No. The premise is "a system is conceivable that has gits upsides and less of its downsides" and look - facebook is even building it.