Let’s say we left one single very smart guy (not necessarily with the knowledge: they may be able to understand hard stuff when taught it, but not know it already) alone on a copy of the earth. That person is also immortal. Could that person, by themselves, gain back all knowledge, maybe also experimental, or even surpass that is already available to us right now, before the planet gets inevitably engulfed by a sun turning red giant?
By themselves? I’m not entirely sure. Some manufacturing processes would be so difficult or physically demanding that they’d require someone to help. But those would be necessary to advance to the next stage of tech…
No matter what you need to do, if you’ve got enough time, you can build a long enough lever.