There's not an engineering company in the world that designs around requirements that don't need to be met. If Apple never foresaw the need to allow non-WebKit browsers to work with PWA years ago, there would never have been a reason to do extra work for no particular reason. They've said that it would take a major redesign to accommodate these imposed requirements. It isn't a security hole because it works as designed, not as someone else wants more than ten years after they implemented it. It met their needs for a lot of years, and expecting a company to waste time and resources on something that wasn't needed is ridiculous.
When requirements change, engineering companies then decide the risk/reward of whether it's worth doing the work to adapt to new requirements. That process doesn't change regardless of whether it was an internal decision or an externally imposed requirements. If they don't see any value in fixing it, they won't. In this case, they've said that there aren't enough people who use the feature to warrant expending engineering resources for that, rather than allocating those resources for a feature people actually want to use. It's that simple, but yet people are trashing Apple for not anticipating more than ten years ago that some governmental body would force them to do something beyond their original implementation. It wasn't necessary back then, so there was no reason for them to do it.