Correction, MAD has prevented WWIII, so far. Maybe the period between WWII and what will develop into something known later as WWIII is much longer than expected. What if Hitler's mayhem were eclipsed? Is there a country with sizable military might (including tactical nukes) that is willing to use every tool in the shed? The Soviet Union was not that adversary, thankfully. No, not North Korea. Could China be taken by storm by a new charismatic national and cultural leader, who takes a more aggressive stance? Maybe Putin suddenly loses his mind or a successor becomes desperate or calls our perceived bluff.
I can't see either terrorists or nations without a country gaining enough leverage to start a global war before cooler heads can prevail, but who is to say that now-secure and predictable weapons will never fall into the hands of corrupt factions of a legitimate government, or that an unexpected military coup could never be successful anywhere? I hope you turn out to be right that MAD prevented WWIII (for good), but the future is long, and is really just history that hasn't happened yet. The best we can do to protect ourselves is to eliminate as many threats as possible, by whatever appropriate means, and to nullify most of the rest with the ability to destroy an entire country and devastate whole regions if we are tested.
If you acknowledge that there will always be a top dog and a struggle to be number one, do you want to take any chance of losing alpha status? The nuclear genie is out of the lamp, so as much as I hate to say this, we have to maintain our military superiority, and that means keeping our nuclear arsenal ready and usable, able to target nearly any point on earth at a moment's notice, and enough to obliterate any attacker. Maybe I sound like the computer from War Games (maybe I amthe computer from War Games), but having the nukes to dissuade any attacker seems like a good idea to me. Nukes aren't going away, sadly, so you're either dominant or your security is at risk.