It's not difficult to find 46/30 cranksets. FSA has both carbon and aluminum options at reasonable prices that are widely available. There are offerings from Rotor, Sugino and others. It would be nice if the "S" and "C" companies jumped on the bandwagon, but it's not essential that they do.
A lot of gravel bikes will also work fine with MTB cranks, which open up a huge range of gearing possibilities. I don't understand why anyone would go with one of the monster-sized - and weight - cassettes with huge gearing gaps, requiring special derailleurs, or adapters (Roadlink, etc.) that compromise shift quality, when there are much lighter, simpler options that work with normal-sized cassettes. Even on a frame designed for road cranks, it seems better to compromise the chainline slightly than to deal with the downsides of huge cassettes.
Heck, I have one bike with a road triple crank with the outer ring removed that gives me 39/24 chainrings. Most of the time I run it with an 11-25, 10 speed cassette for gravel road use and it's shifts like a dream. It's Campy shifters and rear derailleur, with a SRAM front derailleur. I can easily go up to a 29 in the back if I need lower gearing, without compromising or adapting anything. You could do the same thing with Shimano or SRAM 11-speed and get even lower gearing if you need it, without huge gearing gaps.
Yes, I'm primarily a roadie, but I've fallen in love with dirt/gravel road and trail riding.
I thought in Shimano world you can't mix MTB and road front derailleurs (not just rear) because they use a different pull ratios. So that's not true? With SRAM I think it works, although i'm not even sure if they still make MTB FDs.
What if your frame has a braze on FD mount? Do modern MTB FDs work with that and can the FD go low enough for a mountain crankset?
The taking off a chainring from a triple idea sounds interesting, although you can only get a triple with Tiagra 4700 and below, and i'm not sure if there is a triple for SRAM road stuff.
I don't think chainline is an issue with 135 and thru axle disc frames.
Still, it would be nice if I could just buy a new, up to date complete groupset with the specs I want when i'm building up a bike. Sure, I can get an FSA crankset, but then I need to sell the Shimano/SRAM crankset that came with the groupset and probably loose a bit of money. I know, first world problems, but... 😉