It’s interesting because not knowing exactly how powerful something is feels more realistic. I think the uncertainty has it’s place as a narrative device. One way to mitigate the problem you’re describing would be more liberal use of passive checks, like if the party is facing down a troll, a nature check might reveal it’s a powerful ancient beast. For your 17th level fighter example, a good insight check (or passive insight) would let the DM say ‘dont mess with this guy’ without it breaking immersion too much.
You killed zombie Flanders! He was a zombie?
https://frinkiac.com/caption/S04E05/1192557
Plus
https://memory-alpha.fandom.com/wiki/The_Best_of_Both_Worlds_(episode)
Equals meme