IT would be a big mistake to think that all of Hamas is situated in the Gaza strip and destroying every single bit of it will defeat Hamas.
But all of Hamas isn't in Gaza, and I'm sure that Israël isn't making this mistake.
I do think however, that they can defeat Hamas by destroying what Hamas is fighting for. But that's also a mistake: Hamas is fighting to get all of Israël, not just Gaza. That was the problem with Hamas in the first place.