The sanctions are meant to put pressure on Iran to stop development of nuclear weapons. A regime change is not a likely consequence of sanctions, it's much more likely that the Iranian government looks at the economic toll of it's choice to pursue nuclear weapons development and decides to change course. Without a doubt these sanctions cause much misery - the bulk of it inflicted on everyday people who are not decision makers in the country. But allowing a country that regularly threatens to wipe other countries off the map to develop nuclear weapons stands to create orders of magnitude more misery than economic sanctions.