Well, Hulk is Immortal, even if Banner isn't. It's even been hinted at and outright stated that Hulk would go on living forever, even in the works of Peter David and Greg Pak. So it's not surprising that Hulk/Maestro is still alive millions of years into the future.
However, don't let the synopsis of the issue and the cover fool you. When asked of Hulk's involvement and being King Thanos' personal WMD, Donny Cates replied with a "keep reading". So I would not be surprised if King Thanos was unable to kill the Immortal Hulk and thus instead chose to utilize him as his own personal WMD against any potential enemies.
It's a logical theory and choice.