I don't think Hideo Kojima really wants to continue with the series. The way MGS4 ended was perfect, And I do mean perfect! And the way it ended left really no possible explanation for a sequel. I'm pretty sure they didn't intend to go any further. Actually the way Kojima talks about it, He seems utterly sick of the series himself and wants to quit where it is. But he had a good point when he said "You do not kill a goose that lays golden eggs".
The MGS franchise is needless to say "HUGE" And despite the perfect ending fans of the MGS series don't want to leave the game behind. I for one wouldn't want my favorite game of all time to die out leaving me to play the same old story over and over for the next 30 - 50 years. To many people MGS is the all time favorite, Thus birthing an extremely large demand for a sequel. With that being said, Theres alot of money to be made by doing a sequel. Money controls the influence of the world :) So why wouldn't he make another one?
But you also have to remember that this isn't the first time Kojima swore an ending to the public. If you notice, All of his Metal gear games left a beautiful and perfect blank page at the end of the book leaving the end of the story empty but yet complete. He does this with the intent to completely end the story for good, Yet the demand just gets higher and he is forced to make a sequel again and again. If you think you're sick of the series by now, Imagine how Kojima feels Lol.
However he always pulls through and regardless of the ending from the last game he still manages to go with the flow and make the continued story even more interesting. So my opinion is let it flow. I for one am actually pretty hyped to see how yet again the God of MGS is going to take the bluntly-Closed off ending, Reopen it, And fit it in with another beautiful and impossible story. Hes pulled it off thus far without destroying the ending of his earlier games, I have faith that this one will work out as well. Long live Hideo Kojima!