It is opt in, when you set up your phone it asks if you want to enable Siri or not. You can say no. You can turn off Siri. You can never use Siri and your voice will never be captured.
The ONLY way to improve voice recognition is to compare what the algorithm THINKS you said with what you ACTUALLY said. The only way to do that comparison is to have a person capable of understanding the language in question, listen to the recording and compare it to the output of the algorithm. There is no other way for it to work, not just from a technological standpoint but from a logical one.
For privacy and practical reasons Apple doesn't listen to every single Siri conversation. The conversations it does listen to are decoupled from the associated AppleID (at least thats what Apple claims, they could perhaps be lying, but it would be a stupid thing to do and that would be a bigger issue than the listening). Assuming Apple is being honest about that, then you've got random people listening to random conversations. Which is necessary for Apple to improve Siri. So Siri is listening and sometimes those conversations include personal details, not because Apple linked them, but because the person speaking said them. Theres not a lot Apple can do about that. They can TRY to train the algorithms to recognize personally identifiable information and exclude those kind of clips, but the algorithms ability to recognize that would mean Siri is really really advanced, which its probably not at this point. So from time to time random clips of Siri recordings are going to be listened to that contain personal information, its really not avoidable.
So why not just let people opt out but still use Siri? Well, it sounds like Apple is going to do that, but its a PR move not a product quality move. Its meant to assuage people who are outraged over something they agreed to in the first place and is a natural consequence of using Siri (or similar). So now that they are going to allow it, whats the problem? The problem is the more people who opt out, the smaller the pool of data is going to become, and the less randomized it will become, and the less ability to improve Siri there will be. Now at first, if only a handful of people drop out, its not going to likely have an impact, but the more people who drop out, the more they are dependent on the people who don't. But both groups of people are going to benefit from the work being done by an increasingly fewer number of participants. Paranoid people who overreacted to a non-story are going to become a drain on the system, hopefully not enough to impact the product improvement overall but possibly. So its potentially damaging from a practical standpoint.
If you are worried about Siri recording you, thats fair, some people value that kind of privacy highly and I have no problem with that, I don't have an Echo or an Alexa for that very reason, I don't trust Google/Amazon with my privacy like I do Apple, and some people don't trust any of them. HOWEVER, if thats the case, if you don't trust these systems, then you should not USE them. This is not the case of listening being an optional but not necessary component, this is a case of a necessary component being limited by people who want all the advantages without paying any price and expecting others to do it for them. Its selfish.
So by all means, opt out, its not hard to do, you just turn off Siri. You've had that option since day 1.