What does the delivery method have to do with anything? The numbers of warheads does not change just because you have multiple in one missile.
Also, MIRV is not as widely used as you may think, because until recently, it was limited by treaties.
For example, the peacekeeper, which was a replacement for minuteman designed to carry up to 11 warheads was discontinued and dismantled. So the US still only has minuteman missiles.
As I added in the edit, it is lot less used then what a cursory look may lead you to believe. Mostly only SLBMs really use it.
Regardless, it does not change the number of active warheads a country has, so hitting one city multiple times means not hitting other cities.
Plus, it would take dozens to really cover a city like New York. You can play around with https://nuclearsecrecy.com/nukemap/ to get a feeling for it.
Edit: To be clear, many missiles are capable of carrying MIRVs. Minuteman III missiles can still carry up to 3 warheads. But due to the New START nuclear treaty limits, they are loaded with a single warhead.
What does the delivery method have to do with anything? The numbers of warheads does not change just because you have multiple in one missile.
Also, MIRV is not as widely used as you may think, because until recently, it was limited by treaties.
For example, the peacekeeper, which was a replacement for minuteman designed to carry up to 11 warheads was discontinued and dismantled. So the US still only has minuteman missiles.
MRV and MIRV are standard…and there’s a big difference between one warhead hitting your urban center, and 5.
As I added in the edit, it is lot less used then what a cursory look may lead you to believe. Mostly only SLBMs really use it.
Regardless, it does not change the number of active warheads a country has, so hitting one city multiple times means not hitting other cities.
Plus, it would take dozens to really cover a city like New York. You can play around with https://nuclearsecrecy.com/nukemap/ to get a feeling for it.
Edit: To be clear, many missiles are capable of carrying MIRVs. Minuteman III missiles can still carry up to 3 warheads. But due to the New START nuclear treaty limits, they are loaded with a single warhead.
Source: https://eu.greatfallstribune.com/story/news/local/2014/06/18/last-malmstrom-icbm-reconfigured-treaty/10773351/
https://www.af.mil/News/Article-Display/Article/485611/malmstrom-completes-final-minuteman-iii-configuration/