The other browser engines have other security CVEs. Switching engines on the backend does not materially reduce the total attack surface for vulnerabilities for any given everyday user. Using security vulnerabilities as a primary condition of existence also leads to the introduction of engine oversight, review, and adamant “update or be removed” policy enforcement upon third parties, all of which who will complain endlessly in the press about the draconian App Store policies they would be subject to. I expect a better case will need to be made to convince Apple.