While I would have liked to seen more 156s and 158s in lieu of 142s and 150s, one has to remember the state of the railways and the reluctance of the Government of the time to allow new builds. Hertiage DMUs could only be replaced on a 2 for 3 basis. 2 new cars if 3 old ones were scrapped, so reducing the number of coaches and seats available.
Pacers and 150s we thought up as a cheap and cheerful way of getting Regional Railways (the big loss maker) new trains. It worked and so for longer routes the 156 and 158 were developed, but again, a reduction in the number of coaches was required. A 2-car 156 would replace a 4 coach train (37+TSO+TSO+TSO+BFO [or similar]) on West Highland, Kyle and Far North services.
In the 80s it was hard to justify the longer new trains that we need today. Had growth not been so huge, I could have envisged 142s being on the scrapline now...