I think it's true that you guys have reached a point where no random encounter is going to be overwhelming. This isn't to say that they won't be challenging - there's room for improvement (mainly by learning from your tricks) in the tactical skill with which I play intelligent opponents, and the "number appearing" roll could make some kinds of encounter considerably harder than you've experienced so far.
I agree that this marks a new stage in the development of the sandbox. Previously the threat of balrogs in the wilderness has kept you a little hunkered-down; it's now possible for you to say "Let's travel 150 miles overland and see what's there" or "Let's set out into the astral plane" and be reasonably sure that this is something you can achieve successfully, which I see as an exciting expansion of the horizons.
I don't think this is the end stage in the sense of "the campaign is about to end." It's certainly a mature stage, in that the main challenges from now on are going to be dealing with the consequences of your previous actions, rather than dealing with the impersonal perils of the environment. For example, knowing that you can take two balrogs fighting alone will mean that your enemies will want to construct their attacks on you more carefully! This quality - that you will increasingly be fighting the organized resistance of foes who are aware of your capabilities - will, I think, tend to lead into the kind of end-game play that gets talked about in D&D, where you build fortresses (to keep your tons of gold from getting jacked), establish alliances (so that your well-organized foes will meet well-organized resistance), and manipulate far-flung empires (since you can now draw on resources from across the planet & across the planes).
When we die, we get raised. Even when raising is not possible under the rules of the game, we've broken those rules without any trouble whatsoever.
Wanting to keep a sense of character mortality is something many players have expressed concerns about. I fully agree! The problem is that under the rules of the game, there are ultimately no situations in which it's not possible to bring a character back from the dead. The canonical raise dead spell presents no hazards for characters with a 13 or better constitution; if we counted on that alone to give death its sting, I think we'd become frustrated as all high-level characters tended to become those who rolled well for CON, and only extreme circumstances (disintegration of remains, enemies making off with the body) were able to create a real threat. And although the LBB-only mandate of our campaign makes some other solutions beyond the pale, both the afterlife logic of the D&D cosmos and the presence of scrolls like limited wish in even 1977-era dungeons means that sooner or later there is no barrier to bringing someone back from the dead.
I've sometimes been tempted to create a rules-level fix for this, but since one of the enjoyable aspects of the game has been following the implications of the original D&D as written, I've decided to work with the existing rules governing the powers mortals are given over life and death. The advantage of doing so is that if I decide to make it tougher for your characters to be raised, then I'm a bastard who it's not much fun to fight. If someone in the setting decides to make it tougher for you, that bastard will be fun to take down.
