I sympathize with these worries. But I'm not too worried.
Metal Gear Solid 5 will be the latest Metal Gear game. Even if Konami makes another, if it isn't from Kojima, the fans likely won't recognize it as a real Metal Gear game.
And in the end, that's all we need. They can't open the next door without the proper key.
They can't ruin his legacy, this way.