If the last few games have proven anything it's that we need a proper creative cm to replace Santi. Maybe Jack is that guy but you have to plan for a future without him injury wise.
Add a top striker and a winger into that as well.
On top of that there are areas where depth hasn't been tested this season that could prove a problem; if Bellerin gets injured Chambers is not a good enough stand in. Same arguably for Gibbs if Monreal gets injured (I expect Gibbs will leave this summer anyway). And what about Ozil? Our best player hasn't been injured this season, what if he gets injured and is out for 3 months next season? Who provides creativity? Ramsey hasn't shown he is capable, Jack never plays and Santi can't play cam to save his life.
I know you can't plan for every occassion, but I also know us Arsenal fans will bitch and moan in 6 months time when one of those 3 gets injured and is out for 3 months and we have no decent back up, putting yet another title challenge down the shitter.