The Polish government studied the plan in 1937 and concluded that no more than 7,000 families (perhaps as few as 500) could actually be accommodated on the island before starvation and disease set in (the total Madagascar population was 3.7 million at the time). So that’s about 30,000-50,000 Jews max that could actually be deported before it goes from deportation to murder.
That’s barely enough to deport the Jews from one city in Poland, and only half the number of those who immigrated to Israel in 1948 alone. It’s about an order of magnitude less than the Jewish population of Mandatory Palestine.
Even if the plan were fully funded, Madagascar would not even remotely match Palestine as a Jewish homeland, and most deported Jews would probably seek to go to Palestine later.
But what if say the American Jewry funded development on the island?
Remember, Madagascar was an underdeveloped French colony. If the American Zionists were forced to accept that Palestine is not an option, would they (and maybe some other Western governments) send aid to fund health and education on the island.