The replacement system mentioned in the article is already under development and is called Sentinel. Northrop Grumman got the contract 3 years ago. It's not a "from scratch" effort as it's largely based on existing technology. They aren't having to redo all the work from the last 80+ years of rocket and ICBM R&D.
I have absolutely no idea how complex an ICBM system is, but I would imagine it's a lot easier to make with modern technology than with technology from 60 years ago.
I’ve consistently heard that there isn’t too much of a difference between an ICBM and a space rocket.
So I presume the military could “just” buy some modern heavy launch stages from SpaceX or the like, and then strap regular SLBMs to them as their payloads.
Modern space launch vehicles are liquid fuelled because it’s the most energy-dense fuel and you can shut the rocket down if necessary. In SpaceX’s case, you can also refuel the rocket for reuse. But you have to fuel it for a couple of hours on the launch pad.
ICBMs are solid fuel because you can leave the rocket fuelled up indefinitely in its silo.
