This just shows that despite the 1.6M/m budget, you did not have people who could do react correctly. React.hydrate have existed since React 0.4 (2013, that is over 13 years ago) and Gatsbyjs was released over a decade ago around 2015.
But while first render is of course important, now everything else is more expensive with htmx because you have to generate HTML on the server, on top of your usual fetch pipeline. Yes, for small traffic it makes little difference, but once you have >10k/s requests, things start to cost.