Firstly, Reddit is closing the stable door after the horse has bolted- they were open enough for long enough for most serious players to have most of that data.
Second, LLMs are poisoning the well. Reddit's content isn't going to be good training data if it is itself the output of LLMs and that is going to happen.
But finally and most importantly, it's not good! The open internet isn't great to train LLMs. A really important basic element of creating good models is training them on good data, and data from reddit as a source is terrible! Reddit as a source is about on par with just scraping the open web. If training data does become valuable you should be looking at places like the New York Times or scientific journals - places that have large volumes of quality training data.
I guess the ideal would be training with some kind of source discriminator or internal source quality parser that allows an LLM to be the chill funny guy who can still school your nerdy ass in distributed network architecture.
The first explains the answer, the second translates the answer into layman, and the first validates that the translation is still mostly correct.
Compare it to other forms of content - news sources and journals which are extremely well curated, digitized books - a massive back catalogue of well edited and well structured data, Wikipedia - extremely high quality.
I would put it this way, training an LLM on reddit is like teaching a child to play piano entirely by exposing them to freeform jazz.