Why just nuclear weapons?
Shouldn't the scope be wider, say all weapons of mass destruction? But even then, conventional weapons become weapons of mass destruction when used in massive quantities (e.g.: massive aerial bombing during WWII).
So in a world without nuclear weapons, wouldn't state actors just go back to doing things nuclear weapons do but with conventional weapons?
You're right that WWII conventional bombing campaigns visited total destruction on cities to an extent that sometimes rivaled or surpassed what happened in Hiroshima or Nagasaki. But there are two key differences.
One is that those campaigns required hundreds of bombers and a near total effort from the Allied air forces to destroy one city. With nuclear weapons, that turns into hundreds of cities destroyed at once.
The other is that nuclear weapons have become much smaller and much more powerful. A conventional bombing raid could match the devastation of Hiroshima or Nagasaki, but could never come close to the devastation something like a W88 would cause to a major city. The bombs used on Japan barely fit in the largest bomber of the era, but a modern bomb can fit a dozen on a single missile.
A single Ohio-class ballistic missile submarine carries nearly 200 bombs. It could create an order of magnitude more destruction than all bombing raids of WWII combined (including the nuclear ones) less than an hour after being given the order. The US Navy operates 14 of them, and that's just one weapons system operated by one country.
Conventional weapons cannot do the things nuclear weapons do, not even close.
One guy in N Korea with a nuclear weapon at his disposal vs Cuba and no nuclear weapon and we can see how difficult it is to imagine Cuba posing a real destabilizing threat vs N Korea.
W/o nuclear weapons, N Korea, in order to invade S Korea and overwhelm it would need millions of troops clothed and fed for months, logistical support, etc. A couple of bombs and the same is achieved without spending billions a month in maintaining troop and armaments levels.
The fact that some of us are able to at least try to limit their use is great considering humanity's track record...
Of course this is all generally speaking, as each class of weapons have effects that last shorter or longer than others within even the same class. e.g. nuclear air-burst has less fallout than a ground-burst. Some chemical weapons break down fairly quickly and become inert, while others linger, etc.
Then you have to consider cluster munitions and land mines, which while apparently conventional, are quite indiscriminate and also can linger for decades. These devices have also been more successfully restricted than other conventional weapons.
To clarify, it appears we will stop using such ordinance which leaves >1% un-exploded as of next year. [0]
Considering such devices have hundreds of individual munitions, that is still not something I would consider "safe".
[0]http://www.acq.osd.mil/tc/treaties/ccwapl/DoD%20Policy%20on%...
Residual radiation and fallout will hardly affect the land if the warhead is detonated high in the air like in Nagasaki and Hiroshima.
The fallout relative to the explosion radius is also smaller with a thermonuclear weapon. The number of people that die from higher cancer risk due to radiation is actually lower with a typical thermonuclear weapon because the lethal blast radius is bigger.
So I don't think the lasting effect on the population is a good argument against nuclear or at least thermonuclear weapons.
My view is that nuclear weapons are really dangerous because you don't need a lot of resources to deploy them and the high density of destructive power.
It is much easier to "lose" 1MT worth of TNT with a nuclear warhead than with conventional bombs. It is also easier to deploy that 1MT worth of TNT if it is just one bomb. A mistake with a nuclear weapon can also cause a nuclear war. Now it isn't a single nuclear warhead against 100k conventional bombs, it is thousand nuclear weapons against maybe a few million conventional bombs.
[0]https://en.wikipedia.org/wiki/Bombing_of_Hamburg_in_World_Wa...
[1]https://en.wikipedia.org/wiki/Bombing_of_Dresden_in_World_Wa...
[2]https://en.wikipedia.org/wiki/Bombing_of_Tokyo#Operation_Mee...
There was a post to HN earlier this year of a site that would estimate damage and allow you to customize the yields, burst-height, prevailing winds, etc. I did not bookmark it as the results are too terrifying.