Maybe last season, or rather last season's success happened earlier than they had planned? And despite the fact that the club qualified for the Champions League, the management is not ready to change long-term plans because of this. And it's too early to judge whether it's reasonable or not - we have an example of Chelsea which spends a lot of money but achieves absolutely nothing.
Yes and I am not saying that Newcastle is making a big mistake here, but I think they could have been a bit more bold in the transfer market. The means are there and maybe they are throwing away some momentum now if they fail hard this season and so far it isn't looking good. It's still not catastrophic, but we know how quickly this can develop into a whole that they can't dig their way easily out of.
I don't know how profitable Newcastle is at the moment and how much financial margin they had, maybe you are right and they should have built on their success. But we don't know what's in the head of management, so it's hard to judge. Maybe they just want to be profitable first and see everything else as a bonus (the Arsenal model that seemed to never really start fighting for titles again and would just keep making money on transfers).
Right now, financial situation is not bad at the Newcastle United with they are back into top 15 clubs of the world after change of management and having place into Champions League give them good backup even right now we have no official statements about profit or loses, but they are doing good work with management is having long term plan for the bringing this club at the top of UEFA clubs.
But we have still few wired things which was not expected because as they were qualified for the Champions League we have few rumors about transfers like Neymar and Mbappé were also in limelight but suddenly all ends without any solid buying for this big event, and now they are having the toughest group as well which is surely not ideal situation, but we can expect good performance from them with right now they are having three matches losing streak in Premier League.