But we're not buying him for the rest of the season, we're buying him for the next few years. So if he proves to be better than what we have, but ultimately not good enough in 18 months' time, that won't have been a good use of resources. If we had limitless money then there'd be no risks, but you can't ignore the fact money is limited and his signing needs to be looked at within a broader transfer strategy, rather than a one off impact buy.
We're not talking about getting Henrik Larsson on a 2 month loan. Only 2 midfielders have cost €80m+ before, and it's debatable whether the clubs that bought them would do the same again this time round (Pogba and Coutinho). It just isn't true that paying that amount of money for this player is a no brainer. It might be the right decision but it isn't mental to have reservations about it.