It will depend. Since we don't have any way to know how this would work under Apple's system ahead of time. It's all just guessing. BUT, if the feature existed. I'd need to know how it's secured. To a level of comfort that I can or can't live with. Is it a deal breaker, meaning I jump ship and go to Android? Again, it depends. If I can live with it (secure enough), then no I don't jump ship. I'm not a fan of Android mostly at all. I think it's hideous, and would rather not waste my time tweaking it on end to my visual satisfaction.
If they can't secure it enough. I may (just winging it here shooting from the hip) severely limit how I use it. I may not keep my work (enterprise) email on it anymore. Since having that AND my personal email on my device may no longer be a good idea. I may move some of my communication to another app. I may switch which browser I use. Again, all depending on what side-loading entails. What kind of access (potentially) does 3rd party apps have access to. GPS location, photos, contacts, bio-metrics, etc.
I know you will say you can just keep the feature off. However all locks have keys, and when they don't have a key they get broken into and cracked. It's only a matter of time (I'm guessing 5 minutes after launch). There will be plenty of nation states and hackers looking to crack that side door to have full access to everything.
To add to this. Would I switch ultimately? Guessing no. I'd limit my use to just what I need my phone to do.
I'd not purchase yearly as I have in the past on new devices. I like new devices.
Apple for me has the better eco system. I wouldn't want to give that up at all. Limiting my exposer to minimize my security foot print would be how I would have to live. My choice has been taken from me. But, I can still avoid Android while longer.