It is understandable to worry about Atalanta future with important players and coach Gasperini leaving as such changes can really hurt team. Your doubts about Ivan Juric are fair especially since he seemed to struggle at Roma before and some coaches like Ranieri make instant positive impact. However temporary job is different from permanent one where coach has time to build their team so Juric might have grown as coach or simply be better fit for Atalanta specific situation making their adaptation under new leadership something interesting to watch.
We've seen teams make major changes to their squads, and this is what happened to Atalanta. However, the difference is that they sold star players, and the departure of the coach will certainly have a negative impact on their performance next season. Another factor could be the team's financial instability, which has led management to choose to sell star players and buy others.
Given other teams continuing to build their squads, we're certainly not confident that Atalanta will be ready to compete for the title this season. Retegui departure has had a negative impact on Atalanta attack. I think it will be difficult for Atalanta to find a replacement with Retegui skills.