When NX took over in 2004 it could be argued little investment was necessary at the time - the 360s obviously were almost brand new, the 315s were a bit dated but were at least refreshed to make them easier to use, ditching the annoying pneumatic door controls, the 321s hadn't that long been refreshed so were in reasonable shape.
What was done, however, (acquisition of 90s and Mk3s and the 315 refresh programme) was done on the cheap - it took until Abellio's franchise, to their credit, to add PIS to the 315s (though they still haven't finished even now) and the Mk3s were, as I understand it (I don't use them) pretty much as-was, despite being a little worn from Virgin's use near end-of-service.
The 379 fleet introduction for WAML, defective door buttons aside, seemed like it was 'done properly' though. Plenty of units were ordered and they were proper new units to decent specification. The 321s from Silverlink though, saw little attention until Abellio took over, admittedly.
Comparatively speaking NX did more for the fleet than Abellio have done, but Abellio haven't done nothing, just sorted out the 321/4s and introduced a set of inconsistent liveries (disregarding the LHCS refresh which has just started). That being said, the stock which has seen little or minimal attention is far more in need of it now than it was then.
321s still running around in FGW's late-90s interiors in 2005 was fine. 2015, they're getting on a bit. Despite that though, they're perfectly serviceable inside, and just cleaning the windows and floors as well as changing a few seat covers would be plenty sufficient. I have to say I very rarely board a 321 and think 'ugh, look at the state of it on here'. I do for the 360s though, simply because I also use South West's 450s. The difference there is night and day.
