I'm a free software nerd and I run LineageOS-sans-Google on my mobile device. I get the vast majority of my software through F-Droid, which works like a GNU/Linux distribution - they build all packages from source and sign with their own key. Only time I would sideload a package is if it's not available in F-Droid (because it's proprietary - although I've since found out about Yalp and use that instead) or if I'm building it myself. It works well enough for me.
I think the objectionable part of a walled garden, at least to a free software nerd, is the walled part. I appreciate the option of being able to step outside if I feel I need to.
"In 2016, the annual probability that a user downloaded a PHA (potentially harmful applications) from Google Play was .04% and we reduced that by 50% in 2017 for an annual average of .02%.
In 2017, downloading a PHA from Google Play was less likely than the odds of an asteroid hitting the earth."
Source: https://source.android.com/security/reports/Google_Android_S...
Is it? What do you think billions of people using Windows have done for decades?
I think asking consumers to evaluate risk of installing third party apps and manage permissions is insane. My phone runs iOS. My family the same. The volume of malware in the Play Store alone, much less in the sideloading community is mind boggling. If I ran Android I would only install software that I had written myself or come from a trusted vendor, and only use a browser (probably not Chrome even, but Firefox) for any third party content.
Any anti-walled-garden screed has to grapple with the fact that in the real world, the walled garden protects people from mass data theft and infiltration. Windows used to be like this, before many more users switched to Mac (which is harder, but not impossible) to backdoor. Replicating the internet security environment of the late 90s but on our phones would be a disaster for the technology-using world.