How would I explain this games never changing formula?

[Yurimi]

Yes, and as with most businesses that over diversify, they will ultimately refocus on their core business of burgers...

As it has been an expanding menu for well over a decade at both big chains (mcdonalds, wendy's, A&W, DQ, Burger King etc) and small businesses around my place, I would say that it has been at least somewhat successful as they continue to do it as they have for years.

Edit: To kind of make this a solid point in my area Mcdonalds is (at least for the drive-thru) open all night, between the hours of 4am-11am they do not sell burgers (unless it is slow and they dont mind making you one specially), you only have their breakfast menu which consists of egg mcmuffins, bacon, eggs, french toast, waffles, hashbrown patties etc. This practice is also mirrored by another burger place A&W who also does not serve unless specifically for you upon request, burgers. Mcdonalds a while ago introduced salads that are still on their menu today. A local burger shop started doing nacho's a few years ago and they are probably the best nacho's I can get in the city and from friend who is a server there they are apperently extremely popular.