Critical successes and failures can easily exist while still having the impossible be impossible, that’s the DMs job
Player rolls 20 on an intimidation check despite their roleplay being more awkward than intimidating? Critical success, because it’s funny and entertaining usually to do so and it’s theoretically possible anyway.
Player rolls 20 on a strength check to lift a giant iron gate? You did a really good job of trying, but no, you’re not strong enough to lift something 100x your size and weight
Critical successes and failures can easily exist while still having the impossible be impossible, that’s the DMs job
Player rolls 20 on an intimidation check despite their roleplay being more awkward than intimidating? Critical success, because it’s funny and entertaining usually to do so and it’s theoretically possible anyway.
Player rolls 20 on a strength check to lift a giant iron gate? You did a really good job of trying, but no, you’re not strong enough to lift something 100x your size and weight