From what I understand, updating an mRNA vaccine is trivial. According to this answer, no or small clinical trials would be needed to update the vaccine, and updating production would also be trivial. Why, then, are we still producing the original vaccines if their efficacy is decreasing given the new variants?
Trivial in concept, sure, but not necessarily trivial in practice. It's not about just producing a handful of doses in a proof-of-concept; if you want to produce a different version of a vaccine at scale it means either building a bunch of new equipment or retooling a previous production line to make the whole vaccine. Since only a minority of people are vaccinated world-wide, it makes sense that the original vaccines continue to be manufactured since these are still effective against the variants that are being transmitted (maybe not as effective as they can be, of course). Right now, it's almost certainly more important to get any vaccine into arms rather than the ideal vaccine.
That said, it would be false to state that vaccines to target variants are not in the pipeline. Pfizer announced today, for example, that they are developing a booster to target the delta variant. Presumably other vaccine manufacturers are doing so as well - not everything in pharmaceutical research labs gets reported. Manufacturers have to make cost-benefit analyses about what variants to target. Had they prepared a variant booster a few months ago it might have targeted the alpha variant rather than delta variant, for example; today, it seems targeting the delta variant is more pressing.
I'd also add that, as far as I know, no regulatory agency has declared that no clinical trials of an "updated" mRNA vaccine is necessary, or publicly specified what data are required, so I think that's just speculation so far, although well-informed. If descendants of COVID-19 become endemic like influenza, then it seems likely that a regulatory framework similar to that for the annual influenza vaccine would allow new formulations to be distributed without large-scale testing (like in this Q&A: Flu vaccine paradox), but we aren't there yet.
Here is the best answer that I have found, from Dr. Katelyn Jetelina, MPH PhD, writing as Your Local Epidemiologist
If we need another vaccine, we can do this incredibly quickly. Thanks to the new biotechnology, mRNA vaccines are really easy to alter. Once the minor change is made, only 2 dozen people need to enroll in a trial to make sure the updated vaccine works. Then it can be distributed to arms. Because the change is small, an updated vaccine doesn’t need Phase III trials and/or regularity approval. So, this whole process should take a max of 6 weeks.