I think if Pacers are to continue in service they should be restricted to use on frequent services only, so the lines with only hourly services get DDA complaint trains on every service while some of the more frequent routes with a 15-20 minute frequency can get non-complaint trains on alternate services only.[/I]
What diesel routes are there that meet your criteria of a 15/20 minute frequency?
Most routes which have that for part of the journey see hourly/ half hourly (etc) service at the "branches".
For example, Manchester to Bolton is frequent, but only hourly to Blackburn.
Manchester to Wigan is frequent, but only hourly to Kirkby (and half hourly to Southport).
Sheffield to Barnsley is frequent, but only to Castleford or Huddersfield.
Where do you draw the line?
Hopefully, the stock running on those routes will be cascaded to replace the pacers?
The stock freed up by electrification is going to be mainly longer distance trains (HSTs with EMT/ FGW, 222s with EMT, 185s with TPE, 170s with LM and FSR), with the exception of the Valley Lines stock and the FGW 165/166s (which don't have clearance to work on many lines).
I personally think it'd be hard to get a stock cascade working that ensured Pacers were replaced by units capable of running on Pacer services - e.g. a 158 or 170 might become spare, but their acceleration would be rubbish for meeting the "stop every five minutes" nature of a lot of Pacer routes.
Some Pacers will be directly replaced (Leeds - Batley - Huddersfield sees a few 144s, for example), but this is the exception not the rule.