I think the talk of Mary Poppins 3 or 4 was more of a hypothetical to create hype for Mary Poppins Returns than any concrete plans.
When Returns didn't perform the way they thought, I think those proposals were scrapped.
Poppins Returns nearly broke even in the theatrical window. Which means that by the time she went to premier movie cable channels, PpV, DVD, merchandising, and then Disney+, she was profitable in the end.
The MPR Mary was going to be featured in the UK with the (indefinitely) delayed attraction.
Other Disney IP that didn't (or barely) made a profit in the theatrical window, but have a life of their own afterwards: Hercules; Princess and the Frog; Tangled; Winnie the Pooh; Wreck-it-Ralph.
Indeed, many, of the classic Disney animated films of the 40's and 50's were not profitable or only managed a very small profit in their theatrical window. But they made up for it in a regular series of theatrical re-releases, then VHS, then DVD.
Disney knows how to leverage a well-liked IP that didn't make a profit at first into a merchandising franchise that eventually gets it into the black. Disney plays the long game. And when some 200 million people a year go to a Disney castle park and meet a character from a movie that wasn't profitable... The movie becomes profitable.