I don't think the question should be is there better ways/methods than public domain stuff, but rather does using public domain material have the potential to earn profits? And the answer to that is yes.
I don't have Yanik's stuff, but I've been using PD materials (books/images/videos/audio content and etc.) for a long time in niches and turn decent coin with it.
As I understand it Yanik made a killing selling old PD books on copywriting and sales to marketers. I don't play much in the make money online niche, but don't see a lot of opportunity there for others using PD products since beyond copywriting and sales there's probably not a lot of PD content left.
However, there's a ton of other niches you can find great content for. Anything that hasn't changed much in the last 100 years is a good start, and when you really think about it very little has changed all that much in the last 100 years.
Parenting skills, not really different today... so PD content is still going to be relevant.
Fishing, hunting and Camping? The tools have become more modern but the methods are still the same.
Knitting and sewing hasn't changed.
If you're creative there's a ton of great content out there that you can turn into products in tons of niches almost instantly and make money with.
Again, PD isn't better or worse than anything else, it's just another method you can use to get easy content and make some quick coin with it.
Sorry I don't have the resource to share, but as said above I think I've seen it here before so check out the search feature.
