@anemone destroying the ring destroyed saurons power . he was amassing forces & if they had dropped the ring, eventually he would have invaded & wiped out gondor or whatever . + the ring has tricks it uses to get around & probably would resurface again later - consider it was dropped in the river & found its way to gollumn . iirc
+ gandalf says it's "irresponsible" or something
"'Then, said Glorfindel, 'let us cast it into the deeps, and so make the lies of Saruman come true. For it is clear now that even at the Council his feet were already on a crooked path. He knew that the Ring was not lost for ever, but wished us to think so; for he began to lust for it for himself. Yet oft in lies truth is hidden: in the Sea it would be safe.'
'Not safe for ever,' said Gandalf. 'There are many things in the deep waters; and seas and lands may change. And it is not our part here to take thought only for a season, or for a few lives of Men, or for a passing age of the world. We should seek a final end of this menace, even if we do not hope to make one.'
'And that we shall not find on the roads to the Sea,' said Galdor. 'If the return to Iarwain be thought too dangerous, then flight to the Sea is now fraught with gravest peril. My heart tells me that Sauron will expect us to take the western way, when he learns what has befallen. He soon will. The Nine have been unhorsed indeed but that is but a respite, ere they find new steeds and swifter. Only the waning might of Gondor stands now between him and a march in power along the coasts into the North; and if he comes, assailing the White Towers and the Havens, hereafter the Elves may have no escape from the lengthening shadows of Middle-earth.'"
fellowship of the ring book II chapter 2