Yh, if Allen was taller or bulkier than he would still be in the team but he can get bullied off the ball against physical opposition which is why Klopp has been playing Milner over him. It's a shame because he offers attributes that the team needs and neither Can or Henderson can offer those attributes.
Unless Henderson gets his **** together, he will slowly be phased out of the first XI as Can is likened more by Klopp and he has a higher ceiling and we are in for a CM that will bring qualities that Henderson cannot offer when playing in a midfield two.