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That alone is not Mumbai, this is Mumbai Metropolitan region. The region further extends into more of Palghar and Raigadh with more than enough space.
Navi Mumbai, Panvel, Alibaugh, Pen is where Mumbai city is now expanding under the name of Navi Mumbai.
Between Thane and the tri cities of Bhiwandi and the tri cities of Bhiwandi, Kalyan Dombiwali, Ulhasnagar~Ambarnath, there is enough open space. Industries are also shifting rapidly to south and north of this region, Palghar etc. A lot of spill over in services has went to Pune while in Industry it has gone to Nashik and to the whole Dadra Nagar Haveli Daman, Vapi, Tapi, Valsad, Navsari and straight to Surat.
Mumbai Metropolitan in itself will harbour at least 50 to 60 million people within next few decades hopefully and together with South Gujarat (Vapi to Surat belt) will continue to act as Industrial and financial services hub.
The way BJP Govt has taken to development and redevelopment of core Mumbai and the metropolitan region whether through medium and large scale infra projects, redevelopment projects being made easy, Mumbai is bound to bounce back strongly.
Now coming to the Mumbai port area land that you showed, most of that is settled by slumdwellers, mostly peacefuls, can be freed to some extent only through redevelopment by giving them flats for leaving the land they have occupied.
Obviously future of core Mumbai lies in massive redevelopment, massive ocean fronts development, more and more commercial space being carved out, slum eradication while most population will shift towards Navi Mumbai and all helped both by high costs in core Mumbai plus better infra especially helped by metro and coastal roads, bridges being built by govt.