Regarding signing old players, it's a bit of both maybe. Look at all the top leagues. Who are the top scorers? Old arse players, mostly. Zlatan is the best player in Italy at age 39, rivalled by Ronaldo who's 36. Lewanwowski is 33 in Bundesliga. Messi is 33 in La Liga; Ramos and Modric are 35. Vardy is 34 in Premier League and won the golden boot last season.
Football still isn't half as athletically demanding as most individual sports so there's bound to be more and more examples like this. I don't think the issue is going for older players who are actually good if there's an opportunity to do so. I think the issue is just Willian. Signing a shit footballer is a bad idea regardless of his age; signing one who's starting to slow down doubly so.