No one really went into the reasoning as to why non-productive assets are poor long term investments (as a general rule) but I do agree with that assessment. To me it seems fundamental so I don't really know how to defend that stance other than the basic reasoning that's already been given. Productive assets create and non-productive assets don't. If I invest in something that doesn't create, I'm fighting for my slice of pie. If I invest in a bakery, they just keep making pies. You get a pie, and you get a pie, everybody gets a pie.
This is exactly it. If an asset is non-productive, the only way it appreciates in fundamental value(as opposed to an irrational market blip) is if there is a rational increase in demand relative to supply. Barring a sustained demand ramp, the long-term expectation for an asset like this is that it tracks inflation. On the other hand, the productive asset generates income, and still exists at the end of the day.
You don't want to own pie, you want to own the bakery and the apple orchard. If you think silver is going to be of stable or increasing value, you don't want to own silver, you want to own the silver mine. You don't want to own a giant pile of wood, you want to own timberland so you can sell a steady stream of lumber every single year..
Productive assets can still be poor investments if you buy them at the wrong price or they're badly managed, but even an average assortment of them will reliably beat inflation over time. The same cannot be said for unproductive assets such as precious metals or other commodities.
https://www.macrotrends.net/1470/historical-silver-prices-100-year-chart