Well until they are replaced there are too many of them to fix that problem anyway.
In a way no, but what Northern could have done would be to put fully-refurbished Class 158s (I'm thinking an ATW or ScotRail Inverness style refurb with quality seats at a reasonable spacing, not ironing boards) on Northern Connect where they would fit very well, and the new order being for Class 172-like CAF units for Manchester and Liverpool suburbans so they would be operated with a combination of 150s and new units allowing everything to have a layout conducive to high loadings and quick dwell times as well as reducing time lost on the likes of the CLC by having better acceleration.
That would leave what to do with 156s, but there are probably enough very quiet branch lines to put them on, stuff like Ormskirk-Preston, Colne, the Cumbrian Coast, the Bentham line and the likes, where loadings are too low for the end doors to be an issue, but once you put a PRM bog in a single 153 wouldn't be enough. Or maybe even send them off to ScotRail to form 3-car units which might be better than the 153 plan.