Please note, the point here is that the manuscripts we have that contain the beginning of a gospel all (but one) have a title, which is always, in essence, 'the gospel according to Matthew/Mark/Luke/John'. And I'm assuming those titles were put there by the same copyists that wrote the manuscripts.
If these titles had been made up, why do they all have one, and why is it always the same? If the original copies were untitled, and the titles added a century later, we would expect to have some extant copies (or copies of copies) of those originals which were still untitled, or have a different title.
We also have statements from second century church leaders about who wrote the gospels. These leaders were separated geographically and temporally yet all give the same names for the authors.