The presumption here is that we could have been vaccinating people en masse since sometime in spring. However, I do not believe large-scale production of an mRNA vaccine was possible at that time. This does not seem to be the case. My understanding is that manufacturing this type of vaccine is significantly different than other vaccines previously produced at scale (flu vaccine, etc.) so they needed to build out their factories almost from scratch, to the exacting standards required of medical facilities, which likely took many months.
To the best of my knowledge, vaccine makers have been producing the vaccine as quick as they can, planning for hopefully good results, but knowing they could just trash it in the case of bad results, and they only just now have tens of millions of doses.
The FDA needs to make a fast-track vaccine approval process for mRNA vaccines where the pathway to deliver the mRNA has been pre-approved and the only change is in the mRNA payload. However, I don't have much faith in the FDA.
It seems obvious in hindsight, but such trials do have an important role in this process. Not to mention mRNA vaccines are new and it could possibly just have made things worse.
That's still a possibility, though "rushing" can be somewhat subjective. By strictly adhering to the scientific method/process, we can hope to eliminate uncertainty in this regard. Unfortunately... as the parent points out, that requires cooperation between geographically dispersed parties (some of whom may have priorities emphasizing social norms such as "saving face")
