I suggest the statistics (lower priority groups = under 50s without other frailties) do not support this pessimistic possibility.
In the early days after the first dose, the immunity/resistance will not be present so it would be surprising (statistically) if people did NOT become infected, say in the first 12 days. But after that the trials suggest both increasing immunity and less severe symptoms if infected (and recall the 90% effective corollary: 10% ineffective).
On increasing the delay between doses to allow, in the first two months, twice as many people to be vaccinated (with one dose) and afford 70+% efficacy for the 80+% most likely to die, I think the judgement call by the UK JCVI is the right one. I understand the 'stick to the trial' evidence/protocol - and there may be legal issues/risks in some countries more than others (and of course there's history baggage around in various countries too), but on the balance of risk/benefit, content (happy even) that UK has grasped that nettle and is squeezing the juice. By mid March when the the second doses will need to be rolled out, the manufacturing and supply chains and systems will be (even) better, so that can be done concurrently with keeping the first dose rate of vaccination going for the under 50s.
Oh, and 'thank you'
@midlife - here's the Marsh Family 'Have the New Jab' video for you (and all).
https://youtube.be/fn3KWM1kuAw