The smallpox vaccine is a sterilizing vaccine, meaning you cannot be infected and cannot transmit it once you are vaccinated.
The MRNAs are non-sterilizing, as are many vaccines. They may reduce symptoms, reduce infections, and reduce transmission, but they are not considered a sterilizing vaccine.
Secondly, coronaviruses mutate much faster than smallpox, and have other hard to deal with stuff like asymptomatic transmission.
The only diseases we ever eradicated were polio and smallpox. I think we got rid of the guinea worm too. Out of all the many many diseases we have fought over the years.
The vaccines we have are non-sterilizing - and my understanding is that the reason is mostly because we needed at least something very quickly and sterilizing vaccines for respiratory virus are harder to research. It is not inherent property of the technology as far as I know.
Sterilizing vaccines for COVID seem to be theoretically possible. "Natural" infection generates stronger immune response on mucus membranes as well as deep inside the body and that provides higher protection against infection. There's no reason why that wouldn't be possible to replicate using a vaccine. But nobody succeeded at making one yet, so we'll have to see if it's practical.
Because unfortunately the current generation of vaccines doesn't completely stop the spread - they certainly slow the spread in the weeks after the second shot, but then antibodies start to wane over the next months (which is why booster shots are being administered). Vaccines still protect from hospitalisation and death, though.
If you believe most virologists the only realistic scenario is that the virus becomes endemic, similar to most flu strains or other human corona viruses (which makes vaccination even more important!).
There are many treatments other than just the vaccine that can prevent hospitalization and death too. Some countries like India rely on them daily since the vaccines aren't widely available to them.
If you get infected with COVID there are far more options than "Sit around with a thumb up your ass until you have to be hospitalized and put on a ventilator".
