I agree with most of them, mainly PlatinumDude, Toujours, Jellicentfan1, Rodriguezjames55 and XanderO.
If you ever asked a true Pokemon fan about this, they'd know that 5 regions in one game is too many. Having Johto and Kanto in G/S/C/HG/SS was enough.
Having all Pokemon obtainable in one game would ruin the purpose of the GTS as well as trading in general, like Jellicentfan1 said.
There would also be incompatibility with other games, and it would go out-of-date as new generations arrive, like what Toujours said.