I did a complete Mauser project just for the fun of it. I wasn’t particularly concerned about the “cost” of it, I wanted to build a rifle based on a vintage action.
Of course, i did not have to pay a gunsmith to put it all together, and I stayed within the pressure limits of what these actions were designed to handle., that being a 280 Remington at around 50,000 psi with 140 grn bullets.
But you are correct. If your sole aim is to build a rifle that can take advantage of all of the more modern cartridge designs and velocity capabilities with a wide range of bullets, building a rifle based on a old military action makes about as much sense as taking an old junked out ‘67 Chevelle and spending enough money on it to put the kids, and grandkids, through college.