As other have said, it's forgotten that Northern had a massive investment of stock in the 90s, i.e Sprinters. There were also the 323s around Manchester, 321s on Doncaster to Leeds, infact the only old stock at Privatisation were the rapidly replaced by brand new trains 308s, and a small number of 101s and hauled stock. Trains cost money and have a fairly long life expectancy. Replace a large number of trains in a certain area and naturally they aren't going to get a massive amount of new stock for some time. Give it about 10 years and there will be sections of the South East moaning about their old Electrostars and Desiros.
Regardling the Pacers being unsafe or even "deathtraps", just how many people have been killed or seriously injured in accidents involving Pacers? None. How many people get killed on the roads, often driving in modern cars with plenty of safety features? Loads. Are cars considered "unsafe" or "deathtraps"? No. I wonder how a modern train would have stood up to being rammed by an 87 at high speed? It wouldn't have looked pretty, and I'm sure had it been carrying a reasonable number of passengers there would have been fatalities. Of cource the media with it's "new is good, old is bad" adgenda would have had plenty of things about how the train stood up really well considering and how things could have been so much worse had it been an old train. Plus one proposal for replacing Pacers is "tram trains". You really want to be on a tram when it gets hit by a heavy rail train at high speed? All these extra safety means higher build cost, more weight, more fuel used, less capacity, higher track access charges. Got to be a compromise, or should we just put fares up such that people end up using their cars instead, statistially far less safe than a Pacer.