I think part of the explanation for Little Portugal holding out are as follows:
The Portuguese came later than the Italians (College St. Little Italy was already established a century ago) and are less affluent.
While Little Portugal has seen gentrification, it's lower on the pecking order for gentrifiers than Little Italy or prime Danforth. It's further out than Little Italy, while Danforth has the subway and is more "green" and park-like, and both have more appealing housing stock. The paved front yards thing doesn't really appeal to the gentrifiers.
Plus, as mentioned earlier, immigration from Brazil and to a lesser extent Angola means you still have a younger generation of Portuguese speaking immigrants.
Ward 17 is 22% Portuguese speaking and Ward 18 is 20% Portuguese speaking. Italian speakers only make up 3% of Ward 19. Interestingly, the Greeks haven't really "left" Danforth - or at least haven't gone that far - as Ward 29 is by far the most heavily Greek ward (9% Greek speaking), though they're most heavily concentrated a bit north of prime Danforth in old East York. They've suburbanized quite a bit, but not in a particularly concentrated manner.
Interestingly, I'm curious why the Poles seem to have "ceded" Roncesvalles so easily - it seems only slightly more Polish than College St. is Italian at this point. There was immigration into the 80s and 90s. I'm guessing it has to do with Roncesvalles being higher on the pecking order, so to speak.