I thought he just said that it would be the last in the Metal Gear SOLID franchise, being that Solid Snake's role in the story and universe was done/complete?
Almost every pre/post release MGS4 interview I read translated to there always being a future for the Metal Gear IP/universe as a whole - and that Hideo would always be closely involved with the projects in some form or another.
Either way, cant hurt to have more. I just would prefer more gameplay this time around. I don't mind the massive cutscenes, but the 6-8 hours of gameplay was a real bummer. I expected it to be about 50/50 and it really wasn't.