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.
Newcastle is a now a big team in England and also in Europe ever since they were bought by the Saudi Arabia Prince they became a team to be feared. This is their first time to be in the champions league for over more than 25 years or 30 years where they have not been there. They are in group F and it is not going to be easy for them because of the likes of Paris Saint Germain , Dortmund, 7 times winners AC Milan, Their group is super strong and they will need to excellently perform to top the group.
It seems to me that the situation for Newcastle would be worse if there were three strong teams in the group (including Newcastle). When 4 strong teams compete for qualification at once, sometimes the weakest team can be at the top (for example, competitors share points among themselves and you exchange victories with them or successfully achieve victory + draw). For example, in the group Bayern - Inter - Barcelona - Victoria, the result of the group was decided in almost a couple of games (head-to-head games between Inter and Barcelona). If instead of Victoria there was a 4th strong team, then the options would be much more complicated.