I honestly believe there's a bit of nostalgia glasses going on here.
I'm sure there's some of that. But I don't think there is enough of it to invalidate the observations.
This has not been my experience.
Why don't you make me a list of what each lv eliminates? That way I can figure out where I've gone wrong.
I'm not going to mention kobolds and skeletons and other things that were moderately scary in small numbers at first level but are only a challenge in the dozens fairly soon. I don't mind that part so much. I also didn't list a lot of beasts, like giant scorpion, or monsters that tend to work behind the scenes, like hags. I'll give some examples of monsters that seem like they should be scary as loners (maybe with a couple of weaker pets or so), as something you could run into and find a dangerous challenge. I also tried to stick to more classic examples rather than putting in everything that might fit the bill mechanically.
By level 2, these critters aren't scary:
Level 5: At this point pretty much anything less than CR 8 or higher can be destroyed by a party.
Again, those are few of the loner encounter examples that stand out the most. If you add in something like, "a dozen kobolds/goblins", or "a few bugbears" you can fill in more classic threats that go away.