Yeah, the Piledriver quads are just poor. FM2s wouldn't help since they lose the L3 entirely. These might beat an i3 in multithreaded but thats it. If your going quad at that price...old AthlonIIs or Phenoms would likely be better.
L3 is of no value to the type of workload such setups would do. And old Athlons and Phenoms can be had in some places, that is more dependent on where you live and how spoiled the people are there.
You can build an AM3+ build for the same price as an FM2 build. FX4130 will be faster than the 750K so the point is mute. Also gains the ability to swap for an 83X0 chip at any point in time.
Same as I said above... where I'm from with a 750K and a HD6450 overtake the 4300 alone in price, add to that the savings on FM2 mainboard. I know, I've seen some sales Americans have, packed builds for cheap, but those are rare and inaccessible to everyone. BTW, it's moot, not mute! And the 4300 is marginally faster than a 750K due to higher stock clock. Both are unlocked, thus for none does the stock clock matter.
Looking at NeoXF's posted chart, the 750K is based on the 5800K... nothing wrong in beating the 4300.
As for AM3+ longevity, no one can vouch for it, the same way no one can know how fast FM2 will be replaced. They might live long, and just as well they might not, as mainboard manufacturers like to screw with clients. As an example they don't like to update stock BIOS on old releases (i.e. 800 series AM3+ chipsets) and this makes people scream when they put in a BD or PD chip and the setup doesn't start.