The continents are still continents because of social pressures. To change the meaning of that word would confuse too many people and lead to a lack of communication when dealing with people who don’t know the new meaning of “island”. But no, there isn’t really an objective reason why Eurasafrica is/are continents and other islands, some of which might be on their own continental plates, are not.
Sure, if it were up to me, north,central, and south america would be counted as one island, as well as eurasafrica.
Now, what’s really funky is when canals are taken into account.
Is the Eastern US one (or more!) islands because it is separated from the rest of North America by the Mississippi/Chicago Canal/Great Lakes/Erie Canal/Hudson River? (I say or more because I’m not sure that there aren’t more canal systems there.) (Not to mention the obvious examples of the Suez and Panama Canal, but they are close enough to natural-looking boundaries that the case isn’t so blatant.)
I can’t really come up with a reason to not have it be an island. But Intuition tells me that even if I had my druthers, the Eastern US is not an island in a way that South and North America obviously ARE. But I can’t really objectify it other than an appeal to naturality. But artifical island are patently islands. Hmmm.
If a canal (or a river-canal system) has locks in it, then it doesn’t split the island or continent into two. So the Panama Canal doesn’t split the Americas, because it has locks. However, some canals run at sea level, e.g, the Suez Canal and the Corinth Canal, so they split a continent or island. The Peloponnesus used to be a peninsula, but the Corinth Canal really turned it into an island. Of course, you can go the other way, and build an artificial causeway that turns an island into peninsula.
Continents vs. Islands: Continents have over a million square kilometers of area. Excepting Australia, they all have two or more mountain ranges (i.e., distinct cordilleras), while even the largest islands have at most only one. All continents also have a craton, a stable shield area which is geologically old; most if not all islands do not. There were suggestions some years back that Australia and Greenland might be removed from their present classifications to be dubbed microcontinents – a structure common in historical geology and which they share the characteristics of, but which is not otherwise extant today.