There are maybe three tech companies in the US that have large security groups dealing with persistent threat actors. Apple is one of them. Google is another.
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.