While the list of .30-30 guns seems extensive, a hard look at the price and availability shows it's not good. Winchester? It's not US made - it's a well made Japanese collector priced version, not one someone would eagerly leave to rust in the gun rack of their pickup truck. Marlin? Now Remington and a lot of knowledgeable buyers are avoiding it due to quality control issues. Mossberg? They have their fans, but it's not been a Brand known for a lever gun much in the past - their shotguns were the main product.
Henry's aren't modern, and Rossi is considered a lesser import. When people talk lever .30-30, they are mostly discussing either the Winchester 94 or Marlin 336. What people want to buy are exactly those - enough used ones come on the market to meet the demand as older shooters sell off theirs. In that light, it is a diminishing market.
The .30-30 is keyed to lever action use, and the existing numbers keep the cartridge on the shelf, but the sale of new guns is dropping off radically. Marlin and Henry are the two major makers filling the niche. Where we would have seen lever actions on the rack at a long gun store, we now see AR15's taking up space, and often quite a few more. It's really no different than the offerings from 45 years ago - levers then came in different barrel lengths, furniture, half or full magazine, round, semi or octagon barrel, a variety of finishes and grades of embellishment. Now we have just a few and all the options are seen in AR15's. The market has changed to rifles that use magazines - and after 45 years of being issued as a service rifle, the AR15 is finally beginning to influence traditional buyers. There is a constant demand for carry handle uppers in the old school pattern, just the same as there used to be a demand for half tube semi octagon lever rifles.
Yes, the .30-30 is diminishing on the market - but it's far from gone, and likely never will be. It has a place because of the simplicity of it's layout and design, in the ergonomics of it's use. It's a high capacity firearm that doesn't look like one, and that plays a big role today because of politics.