IMO 45-70 is overrated. Introduced in 1873 it a short, unsuccessful career as a military round. By the 1890's they were pretty much dead. When the 30-30 was introduced, it was considered a MUCH more powerful cartridge, and with 1800's loadings is.
If you conder it overrated, that's your opinion, I however disagree.
A short, unsuccessful career as a military round? What are your standards for short? And "unsuccessful"??
Introduced in 1873, and officially replaced as the first line round in 1892, and continued in service as a secondary round for another decade or so, I wouldn't call that "short.
For comparison, look at the official service life of the other military rounds, AND keep in mind that military service life is NOT directly based on the effectiveness of the cartridge, but ALSO on the rifles it is used in, and also technology, political, and economic considerations.
The .30-40 Krag replaced the .45-70 in 1892. The .30-40 Krag was replaced my the .30-03 in 1903. That's 11 years, compared to the .45-70's 19. The .30-03 became the .30-06 in just THREE years. The .30-06 was "officially" replaced by the 7.62mm NATO after about a 50 year run, and the 7.62 NATO was replaced by the 5.56mm as the primary rifle round, after about half a dozen years. So, looking at that, how does the approx 20 year primary service life of the .45-70 equal a "short career"???
The .30-30 is not, and never was a MUCH more powerful round. It has slightly more energy on paper, a couple hundred ft/lbs or so, (depending on which loads you compare) but what it was, was a round that shot flatter, recoiled less, and came in repeaters that weighed less, delivered as much as the .45-70 or a bit more, and oh, yeah, was smokeless powder. Those factors together are what I think accounted for the .30-30's popularity.
It was never a popular bison cartridge. Partly because it wasn't powerful enough, but mostly because most of the bison were already gone before it was introduced.
The .45-70 wasn't the most popular buffalo cartridge for the market hunters, as they began before the .45-70 was introduced. But the commercial buffalo hunting didn't end until the mid 1880s, and by 1875 Sharps, Remington, Winchester were all producing sporting rifles in .45-70, and from about 1876 on, the most common Sharps chambering was .45-70.
The .45-70 is absolutely powerful enough for bison, in its original black powder configuration. Yes there were other, bigger, more powerful rounds used, most of them predating the .45-70, and lots of the people using them kept on using them, after the .45-70 was available.
45-70 was all but dead from the 1890's to 1972 when Marlin brought them back. Sales were slow for a while, but I see a lot of interest in the last 20 years or so. I'd not be surprised if 45-70 hasn't killed more game in the 21st century than the 19th and 20th centuries combined.
All but dead? your call, I suppose, but every black powder rifle round suffered a large slump in popularity with the advent of smokeless powder rifle rounds. Winchester continued making and selling .45-70 model 86s until production ended in 1936. .45-70 ammo production continued until it was halted, along with nearly everything else, for the WWII production effort. AND, it resumed right after the war ended, continuing to this day.
If not being in the top few new rifle sales calibers means "all but dead" there were a lot of all but dead rounds in use between the 1890s and the1970. Interest in the .45-70 did re-emerge with the approach of its 100th birthday, new rifles were made for the round, and people rediscovered that it was not only as useful as it always had been, but that in newer stronger rifles it was capable of even more. Personally I doubt more game has been killed with the .45-70 in the current 22 years of the 21st century than in the 20th and 19th centuries combined, but if that's your opinion, you're welcome to it.
The milder loadings are comfortable to shoot and suitable for deer and black bear. You get pretty much the same performance as a 45-caliber muzzle loader which is the smallest legal option for deer in most places.
If by milder loadings you mean the standard factory load that duplicates the original black powder loadings they are certainly suitable for deer, and bear, and many other things. And yes, when you launch a .45 caliber bullet using 70gr of black powder you get pretty much the same performance from a muzzle loader or a cartridge firearm.
The newer loadings will make it a legit moose or elk cartridge. I would want something more powerful for big bear. But recoil from those loads, in a Marlin lever gun, are brutal. If I'm getting hit that hard with recoil, I'd just go with a 375 and get less recoil, and more performance.
With heavy loads, the Marlin does kick. No question about that. But I found it to be acceptable for what I got out of it. There's no free lunch. The .375 H&H I had wasn't quite as snappy, but it weighted a couple pounds more than the Marlin did.
The mid-level loads will knock the snot out of you, but are no more effective on game than 30-06 or similar cartridges.
Nothing is more effective than dead right there, which lots of good rounds will do. There is no "dead +". IF it puts down the game, it puts down the game. Everything else is a matter of personal preference about what works "better" for the individual users.
Lots of folks think they are getting close to 458 WM performance when they get a 45-70. Even the hottest 45-70 loads are well behind 458. And those suitable for a lever gun aren't even close.
The difference is about 300fps between .45-70 max loads in Ruger single shots or Siamese Mausers and the .458 Win Mag. Lever gun max loads run about 500fps or so behind the .458Win Mag.
If you want a "classic" lever gun, decide what kind of "classic" floats your boat. Something from, or patterned after a rifle from the late 1800s, or something from the 1900 to WWII era? Something else? If you're looking for a degree of nostalgia along with functional utility, the .45-70 has it in spades, as do many other rifles and cartridges. What matters is what matters to you, If a Winchester or Marlin in .30-30 fits your needs, get one. If a Savage 99 in .300 Savage does it for you, get that. If you want a .45-70 of some kind, get one. They all work just fine, within the limitations of the loads. rifles, and shooters.
I've had several .45-70s, Marlin 1895 (the first one with the button safety, NOT the Guide gun), a Siamese Mauser, a Ruger No.3 and a T/C Contender. I've loaded from black powder levels up to what the Ruger can take. I know what they do, and don't do, and what they do, they do well. IF you need or want something they don't do, get something else. But don't tell me a .45-70 won't do what I KNOW it will do.