One would like to think that. "Should" is the operative word, here. A person with certain skills can build a very good multi-copter drone from openly available parts. The real question is of doing it on a serial-production scale, at a 3-digit to low 4-digit retail price ... then building the reputation for quality hardware that DJI has earned.
This is only partially true at this time. Currently when one assembles drones from parts here you have two choices, digital or analog. An analog drone has inferior video quality transmitted back to the controller and you use the drone to carry around a high-quality action camera like a Go-Pro, Insta360, or others if you need high quality video or pictures. Most of the components still come from China. I have gone this route in the past.
If you decide to go digital your choices are much more limited with the vast majority of people currently purchasing the DJI O4 Air Unit Series with a camera. What you end up with is something that is basically a customized drone built to your own specifications which likely will be meant to be superior to a DJI drone for the purpose that you intend for it. This option will likely no longer be available after the “ban”.
The biggest problems with either approach for most people is that they do not have the skills or knowledge to do this; and the drone and additional equipment needed typically added up to more than what a fully assembled DJI drone with similar capabilities costs. But if you wanted a customized drone for specialized applications this was the way to do it. There are also a couple of firms who specialize in producing kits and completed drones like the ones that hobbyists put together. There are American companies who are big donors to key politicians who are hoping to take advantage of knocking DJI out of the market. DJI produces around 75% to 90% of the drones which are more than toys sold in the US. The companies which are hoping to take advantage of the vacuum created in the consumer market have not cared a great deal about drones which are under $1000.
The vast majority of the drones that will be sold in this segment of the consumer market will still be manufactured abroad. Many of them will likely be assembled from parts manufactured by DJI. Some will merely be shell companies operated by DJI affiliates. The same is true of companies wanting to sell drones in this segment of the market which are assembled in the US. WE do not currently have the infrastructure to supply the parts needed.