The problem is they are not would be attackers, they're countries building up domestic defense that US would have to preempt ala Cuban missile crisis, and sustain preemption over entire continent, with each preemption legitimizing rational for more build up.

Of course US can try to coerce INF for conventional in Americas, but commoditized conventional precision strike are conventional... and commoditized, it's the kind of product where specialized dual use components may need to be sourced... among millions of TEU traffic, but otherwise local industries can build, like Iran.

There's also no global pariah status for proliferating conventional missiles for self defense and hence accessible to many players, coercion / enforcement would require trying to mow grass to keep capabilities out of 600m people...in perpetuity... tall task even for even US. Especially considering form factor of missiles... i.e. sheltered / hidden, they are not major battlefield assets like ships and planes that needs to be out to have wheels turned.

Ultimately it's not about winning vs US, it's about deterring US from historic backyard shenanigans by making sure some future time when US is tempted, and US always tempted, it would risk half of CONUS running out of energy in 2 weeks.

Like the Iran logic is extremely clear now, no amount of defense survives offensive overmatch, the only thing left is to pursue some counter offensive ability that can have disproportionate deterrence value. The thing about US being richest country is US has a lot of valuable things.

Afghanistan took only 18 years.

For the 20 years war you are probably talking about: I wouldn't call significant civil unrest in opposition of the war "getting bored"