I have one counter-example for you - North Sentinel Island. This is a small island in the Indian Ocean where people live who are completely isolated from the rest of the world and as such are probably the least technologically advanced civilization on earth. They're able to remain this way because it's generally been agreed that we should leave them alone (you can't go within 5 nm, of the island) but India does try to keep an eye on them from afar, presumably in case some catastrophic event occurs that would otherwise result in their demise.

So while I agree that the most likely outcome of a technologically more advanced alien civilization encountering us would be a pretty nasty one ... there's a remote possibility that we would instead be a curiousity to them and that they might treat us as we now treat the Sentinelese.

Note: there has been contact between the Sentinelese and the outside world, some peaceful (I found something about Indian expeditions in the early 90s) some a little less so (the British kidnapping six islanders). But it didn't result in their extinction or being colonized, and they remain isolated.