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).