John says: "Nowadays, however, the warmth extends much deeper into
the water column. So the storm can persist much longer."
This sounds like an interesting and plausible hypothesis. But just being interesting and plausible is not enough. Do you have a reference to share for data about change in temperature profile -- particularly in the Gulf of Mexico? Can you point to correlations between persistence and temperature profiles?
As others have pointed out, there has been a rather significant lack of hurricanes (particularly ones hitting the US) for the past decade. NOAA's "ACE" is the closest thing I know to a measure of both strength and persistence for hurricanes, and it doesn't show any particular upward trend over the decades. https://en.wikipedia.org/wiki/Accumulated_cyclone_energy