Even with that (large) Apple security group, iMessage is difficult to lock down properly, as you note. However, I think that the cost of 0 day subscriptions for iOS vs Android tell a pretty good story: iOS zero day subscriptions sold to intelligence agencies/governments cost roughly $1mm / seat (phone compromised). Android -- $10k.

There are many many decisions along the way that end up with that raw 100x additional cost for iOS security breaches -- value Apple delivers to its customers when they purchase iOS products.

You cannot pick and choose from the outside and know which of your preferred opening-up implementations would impact that cost. My argument is that opening this up is one of likely hundreds of possible decisions that would contribute to lowering that cost of exploit.