I basically agree with everything you said. Sam and Dean have been through so much, it's kind of unbelievable when a human or monster manages to get the drop on them. Also, since Sam and Dean have both died and come back so many times, it kinda takes a lot of tension out of the show. You're never worried about either of them dying, because you know the show'll just bring them right back.

I've always thought that they should wrap up Sam and Dean's story in some epic way, and then focus on a completely new group of hunters. That way, you get fresh faces and situations, and there would be tension since you're not sure who will live and who will die, you know?

That was always something that bugged me too, as well as their complete disregard for the lives of the humans possessed by demons as well as the general power creep of the show. In the first season there was an episode where they captured the 2nd in command to the big bad, and were all set to exorcise the demon when another hunter mentioned that if they do that, the girl being possessed will die (she had fallen out a window). It created this big moral conundrum for them, and even went into the season finale when one of the brothers had to shoot a possessed human beating on the other brother to save him. Fast forward to season 3 when they get the demon killing knife and then it just becomes a stab-a-thon with absolutely no regard for the people they are actually killing. I haven't watched since season 8 so maybe they've addressed this issue, but even then, it's still kind of a major plothole the writers decided was too inconvenient to address.

The second problem, as I mentioned, was the power creep. Power creep, if you're unfamiliar with the term, is used in gaming when there becomes an unbalance with the release of new content. Basically, as more content (be it DLC, expansions, weapon packs, etc.) gets released, the game needs to be rebalanced so it does not become too easy for those who have that new content. In relation to the show, the Winchesters, since about season 4, have steadily received "upgrades" for lack of a better term, to their arsenal. They had Ruby's demon killing knife from season 3, they got the Colt working again, and then they got an angel BFF which almost literally gives them God Mode. Yet despite all that, apart from the Leviathans from season 7, there never really seemed to be a great external threat after season 5 (although that may have something to do with Season 5's baddie being the literal fucking Devil).