Well there is an alternative to just putting in a bigger heavier battery, which is better battery chemistry.
Every year there is at least one brack through in that department. New Ge, Si anodes that last 3-10 store 3-10 times as much energy as current ones.
I still remember that a few years back they said 40 times the energy/volume is possible and it should sell for 2-3 years now. It never worked out. Something just always spoiled the plans. Cannot survive enough load cycles. Nobody has a powerful enough cathode.
Whatever it is there is lots of research and the actual advancements, while there, have been quite poor.
The research proves though that there is still more possible it just takes longer than expected.
Still though current battery lifes are simple choice of manufacturers. Batteries are costly. Cheap notebooks often only come with 48Wh. Better notebooks with 80-97 Wh. The MBP somewhere in between with 60/77 on the 13" and 15".
If they wanted the battery unlike the air is not that huge nor heavy in a 15" MBP. If they rearange the other components doubling capacity with a few cells spread out flat on the bottom or the display panel (which would be cooler and batteries like it cool) wouldn't change all that much. Maybe 4mm thicker.
Apple could easily make a Macbook Pro with a 30 hour battery. It would probably be four inches thick and weight about 12 lbs.
4inches it would be more like 4cm or 1.6 in though the 12 lb seem reasonable.
Double the bottom case and dedicate all additional space to battery would probably end up with 4 times the battery capacity about 1.6in thick and quite heavy.
The better alternative is the stuff that Sony, HP, Dell, Lenovo offer with the battery slices that may add a bit more thickness than a built in solution but you mustn't carry the weight around if you don't need it.
Apple could build some mounting holes into the current design and offer such a slice if they wanted to. 5mm slice would already double the battery life.