The first issue with changing the damage ratio is that it's a game-wide change which won't have uniform effects. Suddenly things that didn't combo will (e.g. Jabs to Smash attacks), moves that were fairly harmless offstage might become deadly semispikes, and combos that used to work might not any more and shield pressure strings that used to allow rolling out in between the hits might become shield breaks.
Assuming "uniform effects" is equivalent to saying "changes stuff" then this would make sense. However, I think changing a setting is supposed to "change stuff". So, kind of a moot point.
You pretty much have to re-examine the whole game every time you change that number, and the choice of number is subjective - why should we play at a ratio of 1.7 instead of 1.8?
So the first thing we should ask when proposing a damage ratio change is "which option is preferred?", but it does not follow that one would have to re-examine the "whole" game "every" time that setting is changed (just for the testing stage would be sufficient), that would be a ridiculous amount of work for the amount of payoff.
The second issue is that every game mode uses 1.0 by default, and some don't allow changing the damage ratio. Changing that number means new players to the scene get the rug pulled out from under their feet and invalidates a lot of the experience they've accumulated playing the game alone or casually.
The default setting shouldn't be a deterrent, if that were the case then it would follow that we should only be playing 2 minute time matches with default items/stages on.
Damage Ratio should be found in Smash => Rules => Damage Ratio. This should be accessible even with first-time used software.
Nothing, therefore, is "invalidated", the same as it was for every Smash game played in tournaments.
The third issue is that I'll bet any thing the game was playtested on 1.0 so even if there's no a priori reason to prefer one ratio over another, you get the highest probability of ending up with a balanced game at that ratio.
A priori is not used to justify opinion (preference of one ratio over another), it is used as an epistemological justification
independent of experience (that experience being testing ratio settings).
Regardless, reason is not inclusive of bets and speculations.
Although I am not an advocate of changing the Damage Ratio on a nation-wide scale for tournament use, I cannot rationally agree with your reasoning against it. In fact I might just pull out my game and start looking at the effects it has on the gameplay.