But that doesn't mean the mRNA vaccines would be failures.
Focusing on the spike was also done because mRNA vaccines against the nucleocapsid protein of SARS-CoV-1 showed evidence of ADE in animals models so nobody wanted to go down that road. That decision to run with the spike protein is very unlikely to be shown to ever be a mistake in hindsight and will have been the right choice (and certainly the right choice given the information at the right time and a decision which produced an entirely successful vaccine).
It is also a really pretty safe bet that this virus will evolve to achieve immune escape just like HCoV-229E does and will start reinfecting everyone again. That won't make this vaccine a failure. And that's a good thing since its more likely that forcing it to evolve to achieve immune escape will force it to made tradeoffs between immune escape and transmissibility/virulence/fitness.