Reddit is effectively just a text-only forum (image and video uploads are fairly recent features, and the latter has never even worked properly) with subthreads and a voting system.
Text forums have effectively been solved ever since NNTP was invented. All HTTP forums are pretty much just centralized recreations of various components of Usenet. With modern databases, load balancing, and infrastructure management, the cost of building, hosting, and maintaining a web forum is effectively negligible.
The only real selling point of Reddit is having all of your forums in one place, with the ability to easily and instantly create your own forum if you want to. Everything else on Reddit is just a random collection of social media features bolted on to appease investors/advertisers/stakeholders, and the vast majority of Reddit users (or at least the ones I interact with) never touch any of them.
The only thing keeping people on Reddit are network effects. Which, admittedly, is a lot, given that’s what’s allowed Facebook to weather its seven quintillion controversies with minimal damage, but a well-designed successor with enough server capacity (this is very important, crashing under the initial migration load has been what sunk a number of previous high-profile “Reddit killers”) could easily take Reddit’s place.