Price of 7750 on newegg right now is 95$. Price of 550T is around 115$. Price of CPU alone ,say pentium g620, is 63$. This CPU can't match 5800K's (stock) performance in either workloads,be it st or mt(it has inferior ISA support too). Pentium is also multi locked and has no Turbo and no SMT. Rumored price of 5800K is 131$. Boards are roughly equally priced. 5800K does not need extreme air cooling solutions in order to be OCed. If you pair it with discrete class GPU that can do hyrbid CF it gives you additional ~40-50% boost (which you won't get with CPU+discrete combination). Overall, APU is just plain better choice for majority of mainstream buyers,it's just logic.
I'm sorry, but it's not that simple,
A10 = $135 (rumored price)
MB = !?
memory (8GB 1600 = 35, 8 GB 2133 = 50)
OK, now let's add the low end VGA, since you can only use turks your choices go from around $50 up until around $80 (6670 DDR5), now the memory and secondary VGA you use will have a heavy impact on the final gaming performance,
now let's try to find an option
Phenom II X4 955 $95 (or i3 for $120, or $60 pentium)
MB= $45 and higher
memory, you can get away with DDR3 1333 and save another $5

graphics: HD7770 = $100
so... if you go with better memory and dual graphics you can end up paying more for the A10 setup, and it will be slower for gaming, with more problems (from the crossfire with 2 pretty different GPUs),
so my point is, if you want to use the IGP, the APU is OK, if you need more performance, go with pure discrete graphics (and in most games even the $ 100 7770 + $50 celeron will beat the APU, but obviously outside gaming.... no chance)
and please, don't talk about inferior ISA and then "mainstream buyers"
it's funny but the worst competition at least for desktops for AMD APUs comes from their cheap graphics cards....
prices are all from newegg,