"what about ONE die body and multiple inserts for it....no adjustment needed that way and you could buy the inserts cheaper than a new die set...?
Your basic idea is sound. Now, if we can get a maker interested in making a "form die set" that way.
"you could load a lot of neck-sized stuff with it?"
That's also a sound idea. And it was actually done long ago. It didn't survive the death of the old Herters company (in the early 70s, IIRC) but Herter's did market a neat "universal neck die" that allowed the use of interchangable neck sizing inserts. I have often wished the supplying company had kept up the production of that and some others of Herter's unique tools/dies but, alas...NO.
Lyman once had a series of PA, "Precision Alignment", seaters that used interchangeable inserts for different calibers (basically the same design as today's Hornady seaters). Lyman didn't sell it that way so users had to purchase one for each caliber OR make the inserts themselves, but it did/does work for a semi- "universal" type seater. With only three bodies, one each for .17 Rem to .223, .22-250 to .35 Whelen, and one for belted magumums or other bodies of that size, I can make inserts to load for most anything. But, I don't make inserts for neck sizing/reforming, that's kinda out of the reach of me and my home shop!.