Interesting topic!
I think the fact that Islam is essentially irrelevant to western living is a huge part of the decline of the mosque.
Gone are the days when people can rally behind a particular movement and find the energy and zeal that the mosque once provided. After 9/11, mosques are reluctant to become rallying zones for various international causes like Palestine and Kashmir and Kosovo of the 90s.
The Salafis have essentially self-destructed themselves into extinction by finding fault with virtually anything and everything that anyone does or tries to do in a mosque. They have become so engulfed in their nit-picking and ostracizing that they have made themselves irrelevant.
From the African American experience, the zeal of the civil rights era has virtually died out as the younger generation does not feel the need to distance itself from American culture.
So ultimately, I think that the youth are a main reason that the mosque is in decline. The fact that the 5 daily prayers are boring and in all Arabic means that they do very little to inspire the next generation. The Muslim youth are dating, listening to music, and befriending the kuffar. Nothing but total isolation—an unattractive choice for all involved—can change that. Khutbahs of an abstract nature do not address the challenges that real Muslim kids face on a daily basis. Trying to comb through and sanitize the outdated teachings of the qur’an and the hadith to make them sound more appealing to the Muslim youth is a problem that Imams across the country are facing. Those who are successful must be willing to engage in a deliberate art of deception and sugar coating that belies what Islam has taught for ages. They then face criticism from the conservative crew who know better and who always have dogma on their side.
The unit of the family is another problem Islam faces. As women are marginalized and the youth are not engaged, the mosque finds itself left to be an institution of old men who are out of touch with western living. Until Islam address the needs of all elements of the Muslim family, its importance will continue to decline.
The Friday khutbahs back when I was in highschool were mostly about avoiding dating, saying merry christmas and acting like the kuffar. They are fighting a lost cause.
Hopefully the new generation of muslims will be more moderate than their parents making it easier for us apostates to come out.