Nothing goes through decades of testing, they go through human trials and then monitor safety at a mass scale for a year or two and then they're done. Same as with the covid vaccines.
Fun fact, the Omicron BA.5 booster isn't tested at all.
The "decades of public use" metric needs to also take into account how widespread that use is.
Now consider how many doses of mRNA vaccines have been given worldwide (1), since the public rollouts began in December 2020. mRNA vaccines are no longer unknown, at all.
1) Billions of doses https://www.bloomberg.com/graphics/covid-vaccine-tracker-glo...