I don't even think they are that great of a design. I had them for years and had extraction issues, feeding issues, galling issues, etc. They have never had the reliability and durability of commercial Mausers and Model 70s, and there is nothing special about the 700 action that allows a more accurate rifle than other actions.
The 700 series was designed as an inexpensive to manufacture action. Because steel quality had dramatically improved during WW-II, the actions were strong. Since manufacturing methods had improved, they were able to make a cheap rifle shoot well. Because they were inexpensive and worked okay they became very popular, and because they were popular lots of gunsmiths learned how to work on them and a big aftermarket developed for them.
The huge appeal for customizing Remmys came primarily because they were inexpensive. You could get a custom rifle built on a 700 for much less than a Mauser or M-70 action.
Nowadays it is not less expensive to build on a Remington. Add to that all the the affordable custom actions that come pretty much trued up all ready and are a superior design, and it's has to make a case for Remington 700s any more.
And if anyone would make a good quality CRF M-70 action clone for $800-$1000, I'd dump my M-70s and use the custom.