I would argue against killing them off for the sake of future cameo's if they ever got around to a reset (not a full reboot) of the show after letting it rest for awhile, but within the Supernatural setting, dead or alive doesn't really matter lol. Would love to see them give the show a 5+ year break, bring in a new cast to pick up where everything leaves off (depending on if how it leaves off allows for it), and sprinkle in the one living survivor cameo's, with the dead brother being brought out/communicated with later via magical means. The show did a really good job initially with how power levels were handled and increased, but it's been pushed pretty far out there for awhile. If they can luck out again on such an amazing primary and supporting cast, get a few good bad guys/story arcs going, I could watch this shit forever.