Wasn't this the same man who said he will make games until the day he dies?
Hideo Kojima can still make video games even if he isn't with Konami anymore.
This is probably the best thing for Kojima. He has wanted to step away from Metal Gear for since Sons of Liberty. His team couldn't carry the torch without him so every time he tried to move to something new, they begged him to come back. Who wants to see their baby in incapable hands? I think it's similar to how Sakurai views Super Smash Bros., except he was welcomed back, something that doesn't seem likely to between Konami and Hideo Kojima (right now anyway).
The main story ended with Guns of the Patriots (yes, I realize Revengeance takes place after) while Phantom Pain fills in the last major time gap in the series' canon. It's fitting to end the series here and for Kojima to move on to other projects. He'll have no problem finding funding and a publisher. Even if he somehow does, Nintendo likes picking up passion projects abandoned by other publishers (e.g. Bayonetta 2, Devil's Third). As long as Kojima wants to make games, he'll be able to make games.