"increase the impact of a success or failure" =/= make a failure into a success or a success into a failure
if I roll a 1 but still succeed with my modifiers, that's still a success even applying this optional rule and the same holds for rolling a 20 but a success being impossible with my modifiers
usually this "increased impact" is just a flashier success or more dramatic failure
"Increase the impact of a success or failure" -> You not only not succeed, something bad also happens when you roll a 1. Because it is not just a failure: it's a CRITICAL failure (as it literally says on the tin:
"Critical Success or Failure". And, yes, a critical failure is still a failure, not a "success after applying modifiers".
"increase the impact of a success or failure"
what's optionally changed isn't whether the roll succeeds or not, it's how significant the success/failure is
when a success occurs on a natural 20, the DM can opt to add some further benefit on top of what the success would regularly yield
in the DMG's example, a 20 is rolled specifically on a *successful* investigation roll, so the DM can choose to increase the rewards
if a barbarian with no bonuses to investigation had rolled a 20 against a DC 25 investigation check, that wouldn't have been a successful roll
when a failure occurs on a natural 1, the DM can opt to add some further damage beyond the consequences of failure
in the DMG's example, a 1 is rolled on a *failed* attempt to unlock a door and the DM chooses to increase the penalty
if a rogue with +15 bonus to thieves tools checks had rolled a 1 against a DC 15 check, that wouldn't have been a failed attempt
without using the optional rule, nat 1s and 20s on skill checks don't impact anything at all, it's just a regular success or failure
using the optional rule, you can turn that regular success a critical success or regular failure into a critical failure
so make up your mind, is autofail on critical miss based on page 242 of 5th edition dmg or is larian falling back to earlier editions?
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The optional rule let's them choose what to do, and they chose to go with how it used to be done.
that was not how it was done in previous editions either
at least since 3rd edition, 1s and 20s only mattered on attack rolls
that's also how it works in pathfinder 1e
and previous CRPGs were compliant with it too
in NWN and ToEE, the engine actually lets you take 20 on open door and disarm traps rolls because the assumption is you could've kept retrying until you got a high enough roll if a success is possible, so you just fail if a success is impossible or succeed if it's possible
kingmaker and wotr also implement it correctly with some checks being just impossible or unfailable with high/low enough modifiers
of course the official stance from wizards is always going to be "whatever works at your table" because that's just expected nowadays, but their intentions don't even matter because what we're talking about are the rules as written
RAW, it isn't legal to autopass on a nat 20 ability check and there's a good reason for that
a 7 INT barbarian succeeding on a difficult 20+ DC arcana check is just stupid and impossible for DMs to rationalize
similarly, autofailing on a check you should've succeeded is just frustrating