Why are sects thought to be short lived?

Barker: Partucularly in world-rejecting NRM's, the commitment is hard to keep up

Niebuhr: Fervour and commitment is hard to pass down from the adults to their children - The means the sect will either wither away or modify its belief to become more world accomodating, allowing its members to live more normal lives, becoming more denomination-like. 

Becker: Described a denomination as 'a sect that has cooled down'. 

However..

Aldrige: Many sects have existed a long time while being just as strong (Jehovas Witnesses)
: They can maintain strict rules (expelling members that do not conform)
: Many have sucessfully socialised children into their sect while converting adults 

Sects that are founded and led by a single charismatic leader may lose support and dissapear once that leader dies out. 

However..

Aldrige: Not all sects rely on a charismatic leader for success

The reasons for people joining sects often cease to be relevant e.g. anomie, social deprivation etc. This is particularly likely in generations following the first generation. 

Barker: As young people grow older the reasons that drove them there disappear and they look for more normal lives. The sect becomes more denominational. 

However..

Wilson: Questions whether sects could ever survive as a denomination as they would have to convert people by preaching outside the sect which would be polluting and corrupting and destroy the fundamental beliefs of the sect. 

In potsmodern society there's fragmentation of beliefs and religion is seen as a personal matter. People chop and change religion and can go spiritual shopping with ease. 
Sects may be short lived as people change taste.