One thing I would add is that you don't need a large list to make good lookalikes off of - some of the best lookalikes were generated from only 5,000 emails.
Lookalikes almost always outperform interest targeting.
Past some point yes. I've seen a list of 5k work. I've also seen a list of 5k fail miserably and get outperformed by interest audiences. I don't think its accurate to say that interest audiences never outperform lookalikes. At some small scales they do.
Also there is a huge difference between a list of 5k emails that signed up and 5k emails that purchased in terms of performance.
You can also combine lookalikes with interest targeting -- once some audience gets saturated, mix a bunch of your past performing segments like a lookalike + interests.
Lookalikes almost always outperform interest targeting.