I think in the past, the issue has been that its impossible to tell a nuclear ballistic missile from a conventional ballistic missile, so there is/was concern we could wind up starting a nuclear war when that wasnt the intent.
I think since that time, some arms treaties have expired or been let go, and also the threat environment is different. How the CONOP for such a system works without accidentally igniting a nuclear exchange with a competitor that may not even be the target of the conventional attack is something that would have to be hashed out.
That’s a major issue. Also, an ICBM is a pretty expensive delivery system just to deliver a conventional warhead.
I was thinking more of the plans for hypersonic, intercontinental-ranged cruise missiles. While those wouldn’t be mistaken for nuclear ICBMs, they would be considered destabilizing for other, obvious reasons, especially right after we’ve withdrawn from the INF Treaty.
Of course, for all the noise they make about us, the Russians and Chinese don’t seem to be too worried about “destabilization” when developing their own weapons.