I think that this isn't the start of a decline for Arsenal at all. Just because they lost points in one match we shouldn't doubt them quickly.
So far they lost points in only two matches and one of them was a defeat against Manchester United in Old Trafford. I don't blame them too much for that loss also because Manchester United were really great at keeping them at the level they wanted. Of course Arsenal will need to improve their performance in big matches to be able to become more serious for the league title. Winning against smaller teams isn't enough for a team to become the champions in the end. Soon there is a Chelsea - Arsenal match also. We will see how good they will be there.
This is just a draw out of many games, I don't think this is the start of a declining run for Arsenal. Just because they lost two points doesn't mean Arsenal will lose the rest of the games ahead. On the contrary I think the team will bounce back in the next game and get a win to stay at the top of the table. It was a tough game for Arsenal against Southampton and I'm not surprised that the game ended in draw
For me, this draw looks like an unpleasant event and the first thought that arose at the same time was "Well, how is it, cannoneers? Why couldn't they try to win?". Of course, this doesn't mean a failure for them, but it gives great chances for Manchester City to overtake them. It is enough for Arsenal to play one more draw or worse, lose, and Manchester City to win the next match and everything is gone. Manchester City will be able to break into the lead, from where it will be difficult to dislodge them. So, Arsenal in the next matches just need to give all the best to keep the 1st place.