I have a welcome for you too.
Phazon being gone for good is unconfirmed. Plus, the creators of the prime series asked if we wanted to see phazon again, so I'm pretty sure it is not gone for good.
Also, why would destroying the source eradicate all phazon in the universe? That just doesn't make sense. Like I said before, Phazon doesn't act like a hive mind. It is capable of self-reproduction, without the aid of Phaaze. Phaaze was likely just the "mother" planet for sending out leviathans, the source yes, but not a battery for phazon. So any other planets, that were never seen in the prime series, that were struck by leviathans and fully corrupted into Phaaze "clones" would still be around, and thus phazon would still exist (in the games that is).
Also, why would destroying the source eradicate all phazon in the universe? That just doesn't make sense. Like I said before, Phazon doesn't act like a hive mind. It is capable of self-reproduction, without the aid of Phaaze. Phaaze was likely just the "mother" planet for sending out leviathans, the source yes, but not a battery for phazon. So any other planets, that were never seen in the prime series, that were struck by leviathans and fully corrupted into Phaaze "clones" would still be around, and thus phazon would still exist (in the games that is).