There are various types of "spaghetti", from the thin ones to quite thick, then vermicelli, spaghetti quadrati, spaghetti alla chitarra etc..Definitely you can't replace spaghetti with fettuccine in all instances, IMHO.

That said, I am team vermicelli (which are thicker). But spaghetti from a good pasta brand (for supermarket stuff, say Rummo, Liguori) are just another thing compared to the Barilla stuff.