The main unknowns aren't around short-term efficacy, but long term immunity. If it turns out that a single dose provides ~80% protection but only for 3 months, whereas the second dose provides strong long-term protection, the one-dose scenario would be significantly worse.
Vulnerability isn't binary. You're not immune one day and susceptible the next. It's much more likely that immunity to getting sick might start decreasing over time, but still remain relatively high. So, perhaps 80% in 3 months, 60% in 6 months, and 30% in 9 months to use made up numbers. Additionally, production is still ramping up so it's highly unlikely that we'll have less vaccine in 3 months than currently. We'll also likely have 1-3 more approved vaccines by then as well.
Unfortunately, we don't know the exact numbers so following this strategy (which I agree might make sense) is somewhat of a gamble. Maybe the numbers are not very favourable therefore the choice shouldn't be portrayed as such a simple one as in the comment I replied to.
