Your analysis is not accurate.
If Moderna and Pfizer had developed adenovirus based vaccines, they would be competing with the Oxford/AstraZeneca and Chinese vaccines.
The Oxford/Astrazeneca adenovirus vaccine is being sold AT COST with ZERO profit. The touted figure is $3-$4 per dose.
The Chinese adenovirus is also likely to be low cost.
So there's no profit to be had with competing head to head with these low-cost vaccines based on existing proven tech.
But if Moderna and Pfizer have a fancy new mRNA vaccine, they can market it as a revolutionary technology and charge more.
Figures of $20, $32, $37 per dose has been mentioned, which is many times more expensive (and presumably profitable) than the Adenovirus vaccines.
For that sort of profit, it's worth going with a riskier vaccine, particularly since governments have already said the companies are not liable for long-term side effects.
Even at $40/dose, total cost is $40 billion/billion people.
Considering the world suffered $trillions in damages, I think it's worth it.
Chinese people were rushing to get Sinovac.