Newcastle have been very active in the transfer market before this, and it is too much to buy new players every off-season. The reason for the decline is that they had too many injuries to key players, there were too many injuries, and they happened at about the same time, so the team felt it strongly.
The opposite. Newcastle United only brought a few players before or during the transfer market yesterday. Only one of the famous players brought by Newcastle from AC Milan with a large fee. Apart from Tonali, Newcastle only brought 4 players. 1 of 4 players besides Tonali is the Lewis Hall who is on loan from Chelsea.
The decline in Newcastle's performance can be seen because many players are injured.
The next season might be different from the current season because Eddie Howe is aware that they need players who are ready to face the tight schedule of playing like this season where they qualify for UCL even though they can't do much.