I think if Arsenal can make a few more suitable signings in the January window they will have more options in different positions in the team and will be able to compete in both competitions without the fear of burning out their key players with the large number and tight schedule of games but I will also advice to focus more on the league where they already have a bigger opportunity to win it than in the Europa league.
Your advice might be better for Arsenal this season, but in general the opportunities in the Premier League are also not that big for Arsenal even though Arsenal are already at the top of the standings. Because Manchester City, which is right under Arsenal at the moment, is also not far behind Arsenal, so that team also still has the same opportunities as Arsenal and I don't think it would be wrong for Arsenal to be able to focus on two different leagues this season (League Europe and Premier League).