That's something that can certainly happen in the lifetime of my grandchildren, if I have any. The problem is that even a two or three foot rise in seas (the current projection for Miami in 2060) requires trillions of dollars across the world so sewage and other nice features of civilization continue to operate.

The short term existential crisis is economic instability and mass migration leading to unpredictable warfare. The long term existential crisis is that south Florida really is gone along with the place where something like half of all humans live, the ocean life that supports billions of people dies out, and increasingly unpredictable weather patterns cause entire ecosystems to fail taking out huge chunks of humanity in addition to war over the remaining resources.