Ultimately, from the little I know about Manga+, you may be right. But as things stands right now the two apps are certainly similar. Both have huge gaps, both will offer some way to buy volumes.They seem like pretty separate endeavours, presumably manga plus will start selling volumes eventually since I think ads + back catalogue sales is the business model for the Japanese app its based on?
Indeed, if what little I understood of this mess is right, Viz would be more of a Netflix thing and Manga+ would be a lot more traditional, with only the last 3 chapters of a series available and possibility to buy volumes.
But both app are not nearly as finished as they should. UI is lacking and individual manga license is throwing some wrench into the cogs something fierce.
If each company doesn't change its mind, the future looks bright, but the path to it is frustrating.