Wikipedia is already dead, they just don't know it yet. They'll get Stackoverflowed.

The LLMs have already guaranteed their zombie end. The HN crowd will be comically delusional about it right up to the point where Wikimedia struggles to keep the lights on and has to fire 90% of its staff. There is no scenario where that outcome is avoided (some prominent billionaire will step in with a check as they get really desperate, but it won't change anything fundamental, likely a Sergey Brin type figure).

The LLMs will do to Wikipedia, what Wikipedia & Co. did to the physical encyclopedia business.

You don't have to entirely wipe out Wikipedia's traffic base to collapse Wikimedia. They have no financial strength what-so-ever, they burn everything they intake. Their de facto collapse will be extremely rapid and is coming soon. Watch for the rumbles in 2026-2027.

Wikipedia is not even in the game you are describing here. Wikipedia does not need billions of users clicking on ads to convince investors in yet another seed. They are an encyclopedia, and if fewer people will visit, they will still be an encyclopedia. Their costs are probably very strongly correlated with their number of visitors.

If we kill all the platforms where content for training LLMs comes from, what do LLMs train on?

Most people went to SO because they had to for their job. Most people go to Wikipedia because they want to, for curiosity and learning.

LLMs will use Wikipedia the same way humans use it