Phone screens aren't inherently vertical... unless we're talking about the official reddit app. The popularity of phones is partly because unlike computers, they have screens that can easily rotate to view different content in the correct orientation it was made for. Watching vertical content on computers sucks, but watching horizontal content on phones works beautifully and we all do it all the time.

I'm concerned that Spotify has enough market power that they will succeed in pushing this trend to the point where all videos in a few years, and perhaps even movies and TV, will be vertical. This shows a misunderstanding of phone technology. Even when scrolling through text articles many people will make their phone horizontal because they can be more comfortable to hold in that position.