SARS became extinct because it wasn't very catching. As far as I know, work is still going on to make a vaccine for it. Comparison with the ridding of the planet of smallpox is spurious. As for covid-19, we have no idea at present what path it will take over years and decades to come. We need to be vigilant and keep on top of the drive to get even better vaccines (and make them available everywhere). If there's one lesson we can take from smallpox and polio, it's that one.