To call this a 'substitute for Champagne' is doing Prosecco a disservice... and is just plain wrong! Made in a totally different way and usually with a little bit of sweetness allowing for a softer style of wine.
I once spoke to a Prosecco producer who said "Nobody makes finer wines than in Champagne, but you can only enjoy one glass of Champagne, you can enjoy a whole bottle of Prosecco!"*
*please note that due to licensing laws I am not allowed to promote drinking a whole bottle of anything!