Irreparable damage to a possible future game state as opposed to obvious irreparable damage to the current game state.
Your opponent could activate a card effect which would change the top card on your deck somehow, irrelevant if you've reshuffled the deck but very relevant if you've played this turn out with the belief that the card you've had a look at will be your next draw.
As long as you're pretty sure of what the top card on your deck is that is going to influence your plays, it could easily also influence your opponent's plays (e.g. keeping the
Solemn Judgment they have for the
Jinzo on top of your deck instead of using it to negate something else which they would've otherwise used it to negate this turn). The entire point to shuffling the deck in the first place is so that you can't just pre-order your cards and so have your entire strategy preplanned, taking a peek at the top card clearly circumvents this.
And also just because it doesn't benefit you doesn't mean it's not damage to the game state (perfect example being shuffling the face down spell/trap cards on the field at the start of your turn, it's not allowed at any time, unless specified by an effect, yet if done at the start of your turn it benefits no-one, other than perhaps allowing you to make them easier for yourself to track, a simple convienience. Or even to place face up equips under the monster they are equipped to.).
I just don't feel that showing the card to your opponent is quite enough, regardless of how this is handled the game state is irreparably damaged, it's really down to a Judge's decision as to which is worse:
Playing out a turn of both players knowing the next draw
or
Resetting it such that neither player knows whats next but perhaps changing the order of the cards in the player's deck, which, to be frank, if properly shuffled in the first place should make no difference to the player's decisions anyway as the order should be entirely up to luck.