At some point, I believe Hideo Kojima has gone on record to say that every single Metal Gear Solid game would be the last one in the series. That might actually be true for the critically acclaimed Metal Gear Solid V, though, as reports surface that Konami currently has no plans to continue the series past the upcoming Metal Gear Online spin-off.
The information comes from French gaming blog GameBlog, which writes (in translation) that "according to our sources... No new Metal Gear is actually being prepared at any level whatsoever. Not even in pre-production, or in the planning stage. Nothing."
Gameblog separately reports that Konami's Worldwide Technology Director Julien Merceron is planning to leave the company over frustrations with the lack of console-focused development at Konami. Eurogamer confirmed Gameblog's information, saying that it "understands both reports to be true."