This is simply not true, their squad actually reached a low point about 5 years ago, after the sales of Nasri, Fabregas. The season where they were regularly fielding the likes of Andre Santos, Squillaci, Djourou, Gervinho, Chamakh etc, scraped 4th place and then culminated finally with the sale of van Persie in 2013 - the last time they sold a key player.
In the intervening 5 years, they have strengthened and improved by spending significant amounts of money. There is no doubt that the likes of Sanchez, Ozil are a level of signing above and beyond the previous years and it has resulted in 2 FA cup wins and 2nd place last season.
This season is a monumental collapse, and probably should signal the end of Wenger's time at Arsenal, but to say it's been a gradual decline over the last 5 years is not true.