It can be circumvented quite easily and Spotify already does.
You can upgrade to Spotify Premium through a website which they advertise during their free tier ads.
Re:Addressing Spotify’s Claims
by gtyras2mrs on Friday March 15, 2019 @04:00PM
(#19403642)
I think Spotify's complaint here is that they can't tell users of the iOS app to upgrade via the website.
Apple doesn't allow that apparently.
If the App Store could be circumvented as easily, Spotify probably wouldn't be raising this issue to begin with.