Newman regarded the “deposit of faith” to have ended with the last apostle. But was realistic enough to observe that Catholics and Protestants alike had “developed” in their understanding of scripture, and applications of it in different situations, in light of tradition. Most Protestants explicitly rejected parts of the pre-Reformation development of the Catholic Church, like Purgatory, but implicitly accepted most pre-Reformation Catholic developments. like most of the Doctrine of the Trinity.
In the late 19th century, there was a little pressure on Anglicanism, and what were later called “mainline” churches, to “develop” their doctrine, without regard to former limits of scripture and tradition. Anglican, and Protestant leaders argued that they could resist unorthodox developments, without any Magisterium. And, for awhile, they were right, as the momentum of scripture and tradition carried them. But Newman foresaw that the momentum would be exhausted, and, without a Magisterium, Protestant and Anglican denominations would be at the mercy of public opinion. This is what we have had since around the 1960s.
In other words, only within the magisteriuml would development be held within the bounds of scripture and tradition. But all the other denominations are developing too. I predict there will be pressure to drop certain Pauline verses from the NT, and add certain other books. In the long run, churches that have no magisterium will find it difficult to reject additions to the NT.