I would imagine we'd have the capacity to make as many vaccines as someone will pay for.
Yes the factories have to be ramped up but that can be done quickly enough. If those low to middle income countries were offering big bucks for vaccines, the vaccine makers would have just put more resources on it.
The only reason it might make sense that we should stop boosting because it is taking them away from people who need them more, is if the vaccine makers made more than needed, or built more capacity than needed, simply because they didn't anticipate hesitancy.
It would be great if the US and other wealthy countries wanted to subsidize vaccines for poorer countries.... I mean, I'd vote for that. That just means we have less money, though, not fewer doses of vaccine.
(and just to be clear, I'm not saying we should overdo it if there is no point. I'm just saying the article and the position of the WHO is not logical)