Irma actually weakened to a 4 over the water before re-intensifying into a 5 before landfall in Cuba. As it moved over the Florida straights post-Cuba, it re-attained cat 4 status before making LF in the keys, but the core became disrupted due to dry air and modest NW wind shear. Watch a radar loop of the storm approaching the keys, it is evident when the core becomes disrupted. It was a half-a-cane from the keys to Naples LF. While the Cuban disruption certainly kept intensity down, it did have time to reorganize and reintensify to a 4 while on approach, but shear and drier air were more an issue as it approached the US as it did reform a beautiful symmetrical core after Cuba, briefly. If the storm had tracked further north towards Miami instead of brushing Cuba, it may have weakened from the same factors that weakened it dramatically from the keys to Naples. Or it may have missed those as it arrived before they did. My money is on Irma being a monster cat 5 if it had stayed north and hadn’t slowed down while hitting Cuba, but there were variables other than land at play that contributed to the weakening before US landfall