But out of curiosity, what would it take to make a monster terrifying to you? Save-or-die?
You have to admit that older monsters had a lot more different ways to mess with you.
For example, based on experience, the most terrifying monster in 1e AD&D was the 1-1 HD vargouille. When my players identified it, the look of horror and terror on their faces was like nothing I'd ever seen. I think it's about the only time I've faced an actual player revolt. Why? Because if you recall, in 1e AD&D the bite of the vargouille didn't do damage - it permanently drained hit points. Death was something you could come back from, but being permanently maimed wasn't.
Compare the sheer terror of the Spectre in every prior edition of the game to what it is in 5e. The Spectre was a monster so terrible that I - a guy that put vargouille's in encounters - hesitated to throw one at the players. Heck, vampires were so terrible that I rewrote them not to do level drain and to do blood drain instead simply because I wanted to use them regularly but double level drain was too terrible of a horror to use as a routine encounter. If they were going to show up a lot, I had to make them less terrifying.
There is this tension here between removing the 'suck' from a monster like a vampire and yet also at the same time giving them some flavor that makes them other than a bag of hit points that is trying to win a battle of attrition with you in the exact same way that say an orc is, just with bigger numbers. This has been a discussion in D&D for a while now, where for example paralyzation had to go because it wasn't fun to lose your turn. But if something couldn't paralyze you, then it would inherently be a less fearsome opponent and things would be all about the damage race.
5e doesn't provide a lot of tools or templates for that. It's busy trying to (for example) make sure that casters don't dominate the action economy with battle field control and save or suck, so even giving monsters spell abilities doesn't provide a real template for making things scary.