No, I would simply not play a system with three central defenders when I don't have proper wing-backs for it, given it's arguably the most important positions to have for a such system to be effective.
You play this versus any good team in Europe and we're getting murdered.
That's not how a system is decided
You analyze Sociedad first, you judge how to hurt them, you devise a game-plan and a formation.
Then you select players from your squad to fill in the role of each position in the system
Xavi and his staff did all that, concluded 3-5-2 is the system to go with rather than the 4-3-3, and then started selecting personnel for each position.
Some great mistakes there like Ferran starting, but not in the winger/wingback position...
You might say it's something that the players are not used to, and I hear that.
But that's a different discussion. Some games are always the point 0 where everything starts.
Even according to your logic, and given our personnel and their characteristics (lack of fullbacks) I predict 3atb to be used a lot this season