I think your concerns are not overblown. Given Manchester City's inconsistent performance last season, it's understandable that we're unsure or worried about this team in the new season.
After a chaotic and crisis season, Manchester City will begin a new season in massive doubt not only from their fans but they already prepared for their comeback in a winter transfer last season.They still have Pep Guardiola and won't have big chances at a head coach position as well as tactics. Pep Guardiola can be challenged more by other coaches in tactics but with him, Manchester City will have acceptable performances and at least can secure their position in top four before thinking of a title.
Furthermore, De Bruyne's departure was quite difficult. But we also have to remember that Manchester City went through a season without De Bruyne, when he was sidelined with a long-term injury. And yes, Manchester City were fine at the time. However, when De Bruyne returned, Manchester City improved even more. But Manchester City didn't dare take the risk of keeping De Bruyne, who is always prone to injury. So, in my opinion, letting De Bruyne go isn't a bad choice. Although, it will certainly be quite difficult to find a player who can match De Bruyne's performance. But I'm sure Pep has a solution.
De Bruyne is old, more sensitive with injuries and Manchester City must move on without this legendary player. His past contributions for a golden era in Manchester City history are undeniable and will be remembered by history but there is no forever party. As contributions of De Bruyne were not too big in two previous seasons, his departure won't affect Manchester City too much.