I don't have any objection to the notion that someone developed a writing system prior to the sanscrit one you mentioned but which we haven't unearthed, or which may have been so impermanent that the traces are gone forever. Old, unknown cities have turned up from time to time all over the place, and it is sometimes a matter of conjecture linking them to the written record.
Homo Sapiens Sapiens, that is us, only go back about 40,000 years at the very most, though. Before that it was all Neanderthals and other homonids, who have only left the crudest form of stone tools, examples of the Mousterian industry. Art, beads, bone flutes, ritual burial, sophisticated tool making, are all more recent than 40,000 years ago, and are from Cro-Magnon and more recent Sapiens Sapiens sites.
However all tribal people encountered by Europeans around the world had art, beads, bone flutes, ritual burial, and sophisticated tool making without also having written language. So, while it's possible that in some lost city written language was developed in the past, say, 30,000 years, but the evidence has perished, it isn't inevitable as you seem to think. That lost city might have existed, but it doesn't have to have existed to account for anything at all. We know of too many diverse peoples who have come from cro-magnon times to the present without developing written language to know that it is, really, an unusual thing to develop.