Among policy and security people, the term they bandy about is Advanced Persistent Threat (APT). They're not wrong; there are a number of recent cases, and these are ongoing, and you've heard of some of them: Volt, Flax and Salt Typhoon and Velvet Ant. There are more you haven't heard about, because only the operators know they exist.

These are networks of controlled devices. They're hard to eradicate, as shown by the fact that they haven't been eradicated: they're still active and being used to compromise systems, including defense and intelligence systems, power systems, financial systems, identity systems, etc.

Is banning foreign gear going to fix this? No. Security isn't a product. It is, however, a process, and in a process you take steps. I think this: we (individuals and institutions) enjoy tremendous liberty in the use of communications equipment in the US and most of the West. Taking that for granted is a mistake. If part of keeping this means the US has to spin up a domestic supply of network gear, or carefully modulate where such gear comes from, then lets do that. Otherwise, The Powers That Be will leverage its concerns into far worse steps.