Some thoughts for this thread, apologies if the ideas offend some, but that's not my intention.
Do you believe reliability of a controlled round feed / mauser action is supreme? (then the forgotten calibers aren't forgotten, they are commonplace)
Do you shoot inside of 350-400 yards? (then the forgotten calibers are probably not just not forgotten, but are probably in their sweet spot where they are working very well as heavy for caliber hunting rounds)
Do you buy high quality used guns rather than buying plastic stuff that will end up at a pawn shop for $150 in ten years? (then the forgotten calibers are pretty common and you're buying $8000-$25000 guns at modern costs to reproduce, even if you only paid $800-$2500 for a sublime example in a "forgotten caliber")
Why do I shoot what this forum typically calls "forgotten calibers"? Because they were typically built on guns that qualify as functional art.
I'm not besmirching the crowd that wants a Howa, Christiansen, Remington, Savage or other such rifle with a Burris, Leupold, or Bushnell optic, its just that those high production choices don't come in the more "gentlemanly" cartridges that have been proven in Europe, Africa, Britain, and in the fine gun rooms of Abercrombie & Fitch, Griffin & Howe, or Von Langerke & Detmold.
Its not an issue of cost, its an issue of preference. The "forgotten cartridges" in fine used guns cost no more than new mass-produced options, they just don't advertise on hunting shows as most of the master gunsmiths that built these weapons have been dead for 50 years.
My son shoots a custom, german made 7x64. Its arguably the most effective light recoil medium game caliber you can buy. (17lbs of recoil to send a 7mm 140gr at 2950fps). Gun would easily cost $6500-$8500 to have built today. It was at a Cabelas for years as a like-new unfired example. I paid around $850, spent $400 doing pivot mounts myself, $450 for a used swarovski optic. About $1600 all-in.
My daughter shoots a Mannlicher-Shoenauer 1903 in 7x57 that was custom built about 100 years ago by a noted German Gunsmith. 7x57 sends a 175gr bullet at 2450fps and only 12lbs of recoil. This caliber has been successful on everything from small game up to moose/elk and brainshots on elephant and hippo. With exhibition wood, german optics, claw mounts, the gun would cost $12000 to build today. I think I paid $850 and will spend another $400 restoring the stock and reblacking some of the metal parts.
These are just two examples of "forgotten cartridges" (according to this forum) however they've been used ubiquitously worldwide for a century. Ammo is readily available online and certainly for handloading. Buying something new at the store is a choice, and that choice usually results in a "conventional caliber" and at the end of the day, its a disposable tool that does a job and then ends up at a yardsale. Nothing wrong with that, that's how most people buy most things. There are profound reasons to buy better quality used goods in "forgotten calibers" though, the least of which is the quality of the gun, but also because the calibers are usually giving you maximum performance at minimal recoil possible. Naturally, if you're willing to take 50% more recoil there is something new that will send bullets flying 10% faster; surely Howa and Savage make them.