Probably when they do something drastic like ban NSFW content or require age verification. Even then something will need to replace Reddit like Reddit replaced Digg. I am not sure if it will be Lemmy necessarily even though it's probably the smoothest transition.
It's not a smooth transition. Lemmy has huge downsides that are played down each time someone says something. It doesn't have a large developer base. The UI is confusing and moderators of certain communities are powertripping. Certain instances need to be instantly blocked for best user experience. Most people won't care enough to 'give it a chance', they are on reddit to kill 15 minutes of boredom, not to setup their experience in a confusing way.
The Reddit IPO is about one thing only, executive pay day. They are trying to get the price as high as possible and get stock into the hands of as many people who actually would like to see Reddit succeed as possible (regular employees, mods, users, etc.). You can't short a stock of no one is holding on to it and you can't sell if no one is buying. No knowledgeable person would expect the stock not to plummet after it is tradable. So they're trying to pad out the numbers with as many of the few remaining loyal Redditors as possible.
Reddit is or was an aggregator, similar to lemmy/xbin. Changes at reddit can deeply affect various instances here. For example, I guess lemmy was quite different two years ago.