The single dose vaccine thinking would likely work for the under 65s as per the phase II & phase III trial results. The booster dose vaccine thinking is because in older people the immune system isn’t so strong & the “reminder” helps to develop a stronger response. However, there were few over 65s in these trials, the current roll out of the vaccines is in effect an extension of those trials & a proportion of recipients can be tested to see what the longer term responses are, bear in mind that the approval was an emergency measure once the main safety checks were completed.
This is an argument that will run & run when the best solution is likely some combination of the two approaches.
I’d expect single dose vaccination for the under 50s to be a thing once the roll-outs get to that stage.