Any particular thing will change, and if you look more specifically at the factors and data predeciting that, then you can make a more accurate guess about that change. But that’s pretty difficult for big, unpredictable things that we don’t have a lot of examples of (like “massive world religions”).

Current data is that Christianity has been on massive exponential growth since the 1800s. There were 2.4 billion in 2020 and that is estimated to go up to 3.3 billion by 2050. So if you’re looking at growth rates, forecasts etc, Christianity is only going to get bigger.

But obviously, a lot of that growth is due to general population growth. And make some pretty big assumptions about world development, ‘progress’ and waning religious belief you could believe that Christianity will boom and then shrink. But there’s not a lot of evidence for that currently. And that’s why I brought up that general guideline, we don’t have any reason to believe that Christianity is going to disappear anytime soon, and we don’t have any evidence that it’ll be here in 10,000 years. So, if I was an immortal onlooker, and I had to make a bet, I’d guess it’d be around for another few thousand years.

Maybe it would help if you explained more about why you think it’s surprising these religions are still around after thousands of years? Religions and cultural items like that don’t generally get ‘superceded’ by new inventions (as happens with technology), instead they general adapt and change to the needs of the culture that uses them. Christianity is the 2000s is massively different from even 500 years ago, let alone 1500 years ago. And in the 3000s it’s more likely that Christianity will be around, but significantly changed, than that it has faded away.