So two things how I hold this view.

First in all those years Hamas didn't manage to make a dent in Israel. Which makes me really think even Hamas leaders didn't expect this attack to go this far. But it so happened Israel was unprepared with its military all busy defending illegal Jewish settlements on the West bank. Maybe they got complacent. Or a conspiracist might say Netanyahu set it all up (but I doubt it). Whatever it was, Hamas did way more damage than anyone expected so things got out of hand

And second in all these years of throwing rockets, smuggling weapons and all the stuff they were getting away with they got no serious retaliation. Paradoxically because Israel's defenses were so good. So maybe Hamas also got a bit complacent and thought they are immune camping there under civilian hospitals and schools.

Indeed. And the reason why such an attack happened is because the people who did the attack were the ruling government body of 2 million people for almost 2 decades, and were left to their own devices to build up a large scale attack, involving 1 thousand+ combatants.

And in a few months, those combatants and military infrastructure will be destroyed and not much of a threat.

It turns out that killing your enemies leaves them no longer able to attack you. And leaving them alone and in charge of a psuedo country, lets them built up the capacity to harm you.