Brazil are in World Cup quarter final, while several other big teams were shown the door earlier. Maybe they aren't pleasent on the eye but they are solid and doing the job without much fuss.
We have Pogba, Sanchez and Lukaku, with some young talents on the bench, while we really lack an out and out winger who provide width on the right. William fits that imo. We even need a player like that more than Barca.
They already had a full stacked attacking lineup upfront when they bought the likes of Alcacer, with all of them breaking numbers and scoring loads of goals. They surely weren't signing Alcacer to take a spot from them. When Baca want a permanent attacking player, they go on and sign either a top name or one of the best talents worldwide, they always did that : Henry, Villa, Suarez, Neymar Coutinho, Dembele..etc. Alcacer doesn't fit that, and even came with than not needing any starting attacking player.
Dalot was bought as a backup for Valencia that if he plays well he will overake his position after a year or something I believe. No one is predicting Dalot to be a starter by the beginning of the upcoming season.
Investing in attacking players as a backup option so they may be starters in the future is a fine principle and I don't disagree of they were thinking that way and have some flops in the way but you need to consider that if you are buying talents as backups you : a) take a risk anyway as there's no guarantee for any talent to prove its worth and b) you aren't hurting your team since you already have a core of a very good team and even if some talents you bought failed to show their worth they will stay as a backup option.