I think most people would understand that, as you've said, keeping the service going is the priority. Whilst I'm sure it would have been much easier to just take trains out and do full refurbs all in one go, there's good reasons for not doing that. Not least that PRM mods, for example, need completing asap; WiFi, new seats, digital displays, etc. can wait longer.
I'd also point out two other things: it's not 2020 yet, so they're not actually late and; once new trains start entering service it should speed up the refurbs of everything else.