Arsenal had failed to maintain their consistency due to injuries and most likely fatigue too as the same players were the ones playing all the time. But I don’t really get what you meant by Arsenal not having a winning mentality. I think they’ve got a winning mentality and that’s why they had been leading the league table for a while.
They had been winning their games and gathered some good points to be a bit ahead. That’s a winning mentality in my book. They had suffered some set back and lost points which led to Manchester City who was hot at their heels overtake them and finally win the league. Manchester City no doubt has a winning mentality and is a superb team.
Leading the standings from the start of the season to the end of the season does not mean Arsenal have a winning mentality. Arsenal lost a lot of focus when Manchester City began to show consistency in their game, and Arsenal have been under pressure since then.
Manchester City actually managed to maintain consistency while Arsenal lost points in a number of matches, so of course this proves that Arsenal don't actually have a winning mentality when they are under pressure. When Arsenal's concentration is disturbed because of pressure from Manchester City or other teams, then Arsenal don't deserve to win the title, even for next season.