I certainly wasn't going to suggest custom for everyone - I've been down that road myself and I can think of literally one person I've actually recommend for considering a custom bike in my life. Moreso as a side note to the absolutely crazy amount of sizes we see Atherton actually offer. It's nice to see Trek offer literally just one of the irregular sizes you've mentioned above (M-L), and there's still companies skipping out on XS, sometimes on XL, and even on S. Tough out there being on the margin!
It makes me glad I'm about the shortest you can get in to most XLs with. If I'm buying a bike a-la-carte I'm going XL but when it came to my own custom bike, it ended up somewhere between an XL and an L (although now it's fallen back squarely in to "L" territory seeing as we sized the bike in late 2016).