Even if Arsenal focused only on the Champions league I doubt if they can actually win it, Arsenal are strong team currently but in the UEFA they are still behind especially in mentality compared to big teams. Barcelona even sadly failed in this last season when they shifted their focused to the champions league since it was clear to them they didn`t have much hope for their domestic league, mentality has a huge role to play here.
Also I don`t think Arsenal squad really lacks experience or trophy mentality since many of them have won top trophies from their previous clubs rather the entire team lack mentality for consistency when it`s obvious that they have everything to function well.
I agree, mentality is also important for having wins in Champions League without strong mentality and having good depth in squad no team can compete into Champions League even we have defeat for the Manchester City which is much stronger and good team in this event. Arsenal are doing good job, but they need more time for having things well organized which are coming because we all know Mikel Arteta are doing good job and having enough things to bring this team at the top level which give them good success and consistency in long run.
No team can go through with just having focus on Champions League because here things are never been easy for any team they need to go through with their full force in all competitions because this is better approach for them and also better strategy for winning in big events UEFA Champions League is never easy to win with average squad and mindset.