Personally I'm still thinking we will get to the 80% of eligible rate. Yes, probably slower and harder than some want but still doable. Just setting a target increases the chances it will be met.
And Israel has already vaccinated over 80% of adults, with the UK likely to do so next month or so, so it is doable and I don't have a lot of truck with the 'it cant be done' brigade. Not saying it will be easy and some will resist but that's where the carrot and stick will come into play. Freedom from lockdown will be one of the carrots but already others are starting to appear.