I'm not an immunologist, but if new variants develop from the last new dominant strain (i.e. a new dominant strain would evolve from delta as a start-off point), would that not still give a delta-specific vaccine an edge against future variants as compared to the original vaccines?
This argument doesn't really track for me. The vaccine industry is not a monolith and normal market competition should provide plenty incentives to produce a better vaccine than your competitors. Not to mention that the researchers and executives there likely have other motives than pure profit as well.