But the "walled garden" on mobile (iOS mostly, but now also Android) isn't really about trusted computing at all. Trusted computing (locked bootloaders) is but a small part of it.
Trusted computing and even remote attestation have legitimate use cases. It's good, great even, that they exist. But just like everything, they can be used against you.
In fact most digital goods that are sold in large numbers via download, are, as far as I'm aware, sold with some form of DRM. Like films and video games. Otherwise piracy would be just too easy. MP3s don't have DRMs, and are still sold (e.g. by Amazon), but those now seem to be largely replaced by music subscription services.
And this might be a reaction to the fact that music piracy is quite easy; if it wasn't, perhaps there would be no Spotify where you get basically All The Music in existence for peanuts. (Note that no equivalent subscription service exists with regards to movies or games: Netflix and Xbox Game Pass have only a limited selection of content included in their subscription.)
Trusted computing and even remote attestation have legitimate use cases. It's good, great even, that they exist. But just like everything, they can be used against you.