Barring any legal violations, "warnings" would likely be fine. Again, I think the best approach here is to strengthen the product instead of limiting or restricting its capabilities and I feel Apple has the talent and resources to make iOS better even after allowing sideloading, alternative app stores, etc. Requiring Apple to do so will give them the push they need to make the product better and more flexible in the long run.
I feel the two are mutually exclusive. You can’t really have a more open platform that is also safer and more secure; it’s one or the other. The freedom to download apps elsewhere also means the risk of downloading the wrong app, replete with the consequences that come with doing so.
If you are saying that apple needs to spend more resources policing their platform after making it accessible to third party app stores, then I question why Apple should be expected to do all this extra work for no financial gain, when it’s a solved issue to begin with and the problem was one imposed on them externally.