It's about as realistic as it gets. From conversations the high end proposed was $20B. My personal feelings it will end up around $11-12B once it's all said and done.
The $4B approved already covers Phase 1 which is a lot. Once pax numbers hit 105MM annum things change. Considering we'll be at 80MM this year, there's room to grow still over the next 10 years. Give a 3% annual increase in pax, 2028 is the year you'll look for the next big build approval.