The Rosetta stone was not so much a necessity as a shortcut -- albeit a tremendously useful and quick shortcut. Any text in an unknown language requires a certain amount of supporting information, context, and clues, for it to become decipherable. The Rosetta stone happened to be a particularly useful and impressive bit of supporting information, as it allowed a whole bunch of breakthroughs all at once. But it's not as though it was the only supporting information that ever existed. In Champollion's time pretty much no one had any experience in deciphering hitherto unknown script/language combinations, so there were no examples for anyone to follow; but folks would have worked it out eventually.

Unknown scripts and languages have certainly been deciphered without the aid of anything quite as impressive as the Rosetta stone: for example cuneiform, hieroglyphic Luvian, and Mayan. The fact that there's no "Rosetta stone" for them means that the decipherment process isn't nearly as quick, impressive, and clear-cut.

The cuneiform case was of course especially important because it represents several different languages, including Sumerian, Elamite, Akkadian, Ugaritic, Hittite, cuneiform Luvian, and Old Persian, all with different variants of the script. Of these, Sumerian is probably not related to any other known language. And for the others, over the 200-odd years that people have been working on cuneiform, decipherment of one language has of course assisted greatly in the decipherment of others; and knowledge of different forms of the language, or related languages, came to be important aids as well (Old Persian is closely related to more recent forms of Persian; Hittite and Luvian aren't as close to any surviving language, but they are both Indo-European; Akkadian and Ugaritic are Semitic).

Another interesting case is the Linear B script. Although it represented a known language (Greek), Michael Ventris (who deciphered the script) strongly believed that that was not the case, and that it was a non-Indo-European language, until to his astonishment he saw Greek words popping out all over the place. So his initial decipherment was unaided by any knowledge of the language that it represented.

I don't know anything about the Indus script, but for the record, Etruscan is reasonably well understood. The script is thoroughly understood. A lot of details about the language remain obscure, but enough is understood to get at least the gist of most surviving inscriptions.

I believe what you're asking is very similar to this.