Maybe that changes the landscape but in the Emirates era Wenger has fielded weakened teams in cup competitions (though less so in more recent years, though I'm not sure if that was due to pressure on him from the "trophy drought") and at times felt like he wasn't too bothered about discarding them for his league and CL campaigns.
It's still a luck game though and it takes a lot of games to win it. You only get into the CL if you win and there's no guarantee that you're going to do that. United have been very lucky with the draws they've had and the EL feels weaker this year than in previous years but that probably won't last.
The league on the other hand is pretty stable. If you put in the performances and get 1st, 2nd, 3rd and most of the time 4th, you'll get in. I hope I'm wrong about him but I really think Wenger is the type of character to prefer that method and I think Arsenal have enough of a bad culture of not giving a fuck that I can see us putting in a meek performance in a R16 game, getting knocked out on away goals and not being too devastated and just forgetting about it.
For example people have blown smoke up Spurs arses this year and that's probably for good reason but look at what they did at the first hurdle in the EL and I don't think that it was because they were focused on the title. They were much closer to 6th place than they were to 1st so will still be worrying about finishing 4th.
We seem to come unstuck on a mental level as soon as the CL knockouts come around so I don't think our mentality will suddenly be fixed once we're in the EL knockouts instead.