I am arguing that the argument is very real and very serious, but in practice, very semantic.
Then, in order to create a bright dividing line where none should exist in theory, the doctrine of assured salvation through a single declarative act of faith is preached
Some Protestants (myself included) do not believe that just because you "prayed a prayer," you are ipso facto saved. As a Calvinist, I don't believe in once-saved, alway-saved because I believe in Perseverence of the Saints - that those who are really saved are discernable by virtue of the fact that they will endure to the end.
All these arguments are very abstract if viewed from the eternal Divine perspective. As long as it is clear that we are talking about the mysteries of predestination and Divine foreknowledge, you can say that the differences are bridgeable, and much is a consequence of the choice in terminology. For example, once faith is understood as faith formed by works of love rather than declarative faith, the 'sola fide' becomes a biblically acceptable doctrine. Once 'sola scriptura' is understood as reading the Scripture in the light of the patristic teaching and in the context in which it was written by the human writer, that becomes acceptable. Once grace is understood as transforming grace rather than a coat of paint, 'sola gratia' becomes acceptable.
Faith as a matter of ongoing conversion and spiritual effort then generates a hunger for the sacraments, charitable work, and self-sacrifice. We see it in all the converts. But this is exactly the point where Protestant indoctrination creates a mental block: "Oh wait, eating this is works! Praying this is tradition of men! Looking at that is idolatry! Confessing to a priest is clericalism!" So, -- and I repeat myself -- while much of Protestant soteriology is a permissible view on the same spiritual reality, Protestant praxis is very damaging. Presumption is just one wound it inflicts. The Zwinglian image of God as author of Evil is another. Radical individualism and self-propelled sectarianism is another. Megachurch mass hysteria is another. These are wounds on the body of Christ.