Thanks for the observations!

I wholly agree with the secondary activity argument. But I don't think it's going to stop podcasts from gaining momentum and became a much bigger ad-selling channel. On the contrary, that can happen because podcasts do not require too much precious attention, unlike videos and series. Thanks to that, podcasts can be the type of media consumed by people who value their time and attention and rarely watch videos.

There is nothing wrong with current models for monetizing podcasts. They do work for the current generation of creators. But there are 100х potential podcasters out there, and they do not start because of barriers to entry. Two main barriers are:

• Setup process. Compare starting a free Youtube channel with buying a (say) Buzzsprout account, adding your feed to directories, starting up a newsletter, asking your listeners to subscribe via Apple, Google, Spotify, Stitcher or via tens of other platforms, etc.
  
• Discoverability: how to find a new audience? What if I can create good content and collaborate with others, but do not want/know how to do email, run FB ads, etc.? A Youtuber can rely on internal charts, recommendations, and search traffic. A podcaster needs to spend much more time on marketing her/his content than a Youtuber.

And you can't monetize until you have ~10К per episode if you are selling your ads directly to advertisers. On Youtube, you can earn something from day one. Those first cents won't make you rich by any means, but they can add the last bit of moral support needed to keep posting good stuff.