While in the Milwaukee area, I had a conversation with a very concerned Catholic about the mission of evangelization. (Fittingly enough, of course, it was Mission Sunday yesterday.) The good man was under the impression that once Catholics came to hold that people could be saved without being baptized Catholics, there was no real reason for anyone to dedicate their lives to announcing the Gospel. In other words, the only thing that could possibly motivate missionaries was anxiety about people's eternal salvation.
I have to admit that what we read in some of the saints' writings could support an impression like that. But what else do we find in those same writings? For one thing, an exuberant love of God: loving God that way, how could they bear the thought of such a good God not being known and loved personally? St. Frances Cabrini used to exclaim to her sisters, "Love is not loved!" So for her, God's sheer goodness was a motivation for evangelization. (Mother Cabrini founded a missionary congregation, so the link is pretty explicit.)
Other missionary motivations include the realization of just how much can be missing in the lives of people who have not heard the Gospel, especially when those people are poor or suffering. In this case, it is an exuberant love of neighbor that motivates evangelization. (Stephen Lawhead's novel "Byzantium" has a hint of this toward the end when a character who has lost everything but gained Christian faith exclaims how much it means to him that God knows personally what it is to suffer.)
And there is a further missionary motivation that does not depend on people being surely damned without the Gospel: people may indeed be saved through God's grace (which has always operated beyond any control of ours!), but Christian faith and the sacraments can help them to avoid or overcome temptations and sin that they may otherwise succumb to easily, and support them in becoming holier, more like Christ, than might otherwise be possible.
So there are plenty of reasons to proclaim the Gospel, even if people "could" be saved without explicit Christian faith.