Newcastle can't be competitive for the Champions League much for now. Maybe they are one of the richest teams in Europe but they still can't make their squad one of the bests yet. It isn't an easy task to convince big names into their squad despite qualifying for the Champions League. They need to maintain consistency in the Premier League first.
Once they convince bigger names with their performance then they will able to bring that kind of players easier. Those players are already playing for top teams in the end. Newcastle will need to give them a reason why to join them instead.
You are right that for now Newcastle's return to the Champions League is already a great development, but to dominate this competition requires a lot of experienced players and quite a strong mentality. It's not easy to be in the environment of top European clubs who are used to playing in the Champions League.
So in the future for Newcastle it will be a challenge to become a team that can be more reckoned with and in the transfer market next season they must be able to recruit big names to be brought in as a form of their seriousness not only in the Premier League but in a more competitive League competition, namely the Champions League.