The only way any one of these franchises are gonna die is if their owners abandon them and leave them to die. Activision (formerly Universal studios interactive) decided to do the very exact this with Crash Bandicoot. This I will never forgive them for.

I don't know about Halo, GT or Mario but COD is starting to die off.

Mario can never die.
Halo could if people get tired of the series, or if the games lose their quality or if people get tired of fps.
Gran Turismo also could if the quality of the games fall.

Maybe after 100 years they can die

Yeah some are too large to die out. If Nintendo went bankrupt today, we'd still probably see Mario games coming out for another 5 to 10 years. Halo would still have books, comics and potential movie deals in place even if Microsoft said Halo 5 would be the last official game in the series. If John Madden were to pass on, short of the family demanding they stop or requesting more money, the EA NFL games would still be called Madden.

Some franchises are too large to die out unless tanked... Mega Man potentially is an example of a company tanking their own franchise by trolling fans with announcements that lead to cancel games.

I can't believe so many people are saying that these franchises are too big to die. Have people learned nothing from history? NOTHING is too big to die.

Gran Turismo maybe, its popularity has plummeted in North America and Japan.

So much so that GT5 has a good chance of matching the sale numbers of both NA and Japan that GT4 got on a console that has a much smaller install base? Not to mention Europe and ROW has exploded in popularity compared to past gt's and is already miles clear in Europe of any gt (6.7/6.8 million roughly based on shipped figures from march compared to 6.8 milllion for gt4, except gt4 has all but stopped selling and gt5 is on track to hit a 4th year of 1m+sales (currently on 700k)

The only franchise that is at risk is Halo as it's story based whereas both GT and Mario (and Sonic for that matter, but they strayed away from their core) are gameplay based and they've been doing it for the best part of 20 and 30 years respectively and have both massively outsold Halo.

How can you put halo in the same bag as Mario ?

Halo franchise is still very young. It's as old as Ratchet & clank or God of War.

There's no such thing as an unkillable franchise. Hell, there's no such thing as an unkillable company.

Anything can be killed by overexposure and/or incompetence.