On malware being distributed through alternate stores, yes. For example:

• https://www.zdnet.com/article/amazons-app-store-puts-millions-of-android-devices-at-risk/
• https://cybernews.com/news/over-9-million-android-devices-infected-with-new-trojan-from-huaweis-app-store/
• https://www.pcmag.com/news/study-reveals-googles-play-store-is-main-distributor-of-malicious-apps (Table VII, "alt-market" row)

The real user benefit will be very small

This is just my gut feeling. It is based on not knowing anyone IRL that has willingly installed an Android app from outside the Play Store, but actually knowing people that avoid it because of the potential security implications.

You have to remember that the vast majority of smartphone users are not power users, and not the people who hang out on these forums. While something may look attractive in small circles like these, there are many other factors to consider when targetting the entire userbase.