Non-life threatening pretty well describes 5E, which, while I rather like the edition overall, is one of my primary criticisms. I feel as though a combat encounter that doesn't risk the lives of those participating isn't particularly thrilling or fun. It's more or less an encounter not worth having.
I agree, but I don't see it as a problem because the game doesn't require you to have boring, Medium fights. My game is combat-light, and fights, when they happen, range in difficulty from trivial up through Deadly x10 or so, but the average "interesting" fight seems to happen at around Deadly x3 to x4. I tend to skip over fights that are less than Deadly, or treat them as basically social encounters.
For example, the PCs were on a floating island that they wanted to use for a space colony. They were trying to clear off the dangerous fauna so they could bring in settlers, and I knew there were about 80 ropers and 70 phase spiders on the island. Rather than play out 50 easy fights over the course of two weeks game time (and maybe about that much real time), I offered them the chance to do some fights where the PCs roll everything at disadvantage and the monsters roll everything at advantage (which stacks with "normal" situational advantage for double-advantage against prone targets, etc.), and then handwave 5x or 10x (I forget) that many monsters in other fights. Essentially, I would let them play "this was your unluckiest fight, the one where everything went wrong." They fought five phase spiders at once that way, and when I counted the adjusted XP afterwards I found that it was already a Deadly fight before you added in the universal advantage/disadvantage... the PCs came
this close to a TPK at least twice during the fight (both times when the spiders attacked the spelljamming ship's helmsman and came within a single initiative roll or saving throw of inflicting 20d6 falling damage on everybody on the ship). That was a fun fight for everyone at the table.
Personally, I think Liches are underrated as end game fights. A Lich who has 9th level spell access and centuries to plan contingencies and defenses as well as gather weapons and allies for when those things fail. I mean just look at all the spells that state "If this spell is cast in the same place for a year the effects are permanent" and realize a Lich and it's allies could have had 300 years to build defenses.
If you're planning to use a Lich as an end game fight, you have the grand opportunity to use him as a midgame fight too... multiple times. Each time he "dies" learns more about the party's tactics and revises his plans, and each time the players hate him more and more. Phylacteries are awesome.
The trick will be preventing him from wiping the players out early on. Maybe find a way to make him cocky in the first fight, attempting to
Cherry Tap them out of boredom. "I will kill you with nothing but this mundane longsword, which I'm not even proficient in." The next time, he can amp it up and permit himself the use of a single spell. Maybe he's not even trying to kill the PCs at first, he's trying to "conserve" them for further challenges. (I do this when I play Master of Orion II--attacking Antares but careful not to kill them all and win, which ends the game.) As a special treat he can let himself kill
one of the PCs. For a normal villain this would be irrationally risky behavior, but a lich can afford to indulge himself. For the final battle, the lich actually takes the gloves off and plays to win.
