Why would anyone be opposed to de-extinction?

[Scootaloo]

76. Untrue, actually

We don't have an environment for them to live in

Even if we assume that "mammoth" means "woolly mammoth" (there are a few other "recent" species contemporary with man) we actually have plenty of woolly mammoth range. They weren't a niche species, and basically spread to whatever environments in the northern hemisphere didn't already have an elephant. Steppe, taiga, deciduous forest, even the subtropical coastal plains of the southern US (they were plains at the time anyway) harbored woolly mammoths. Were it not for the asiatic elephant and the columbian mammoth, they probably would have lived all over Eurasia and the western US as well.

nor the type and amount of fodder for them to eat

Again, these weren't giant pandas. Know what woolly mammoths ate? Plants. Nothing specific, just... plants. If it grew in dirt, they crammed it in their mouths. We know this because we actually have woolly mammoth stomachs full of exactly that. We're not exactly seeing a shortage of alder and sedge grass anywhere.

nor the right type of predator to control their numbers

I'm curious - what predator, exactly, do you think that would be? For a moment, imagine we're talking about a creature that is, as far as we can tell, a hairy analogue to the African elephant... because it is. What eats African elephants? Well, there's some desperate lions in the kalahari who'll occasionally have a go at an elephant, and crocodiles will grab a baby elephant just as they will anything else. The Asiatic elephant's young were apparently occasionally taken by tigers, but that was rare. I think in the case of the woolly mammoth, the only credible predators would have been the Siberian tiger and the short-faced bear... and even both of them would have only targeted opportunistically and were never a serious factor. Essentially the woolly mammoth had no real predator woes. The biggest causes of mortality for them were probably falling into large holes, disease, and fights between bulls. Which makes sense for a species that has a two-year gestation, a five-year weaning, and matures at the age of fifteen.

Mammoths and many other species died out naturally with the end of the ice age.

The end of which ice age? Woolly mammoths (to say nothing of other mammoths) lived through several ice ages. The one they didn't survive was the last glacial retreat... which saw the introduction of a certain tool-using, fire-making primate into mammoth habitat.

These animals arrived specifically to fill a niche the ice age provided and which the planet no longer has.

Given that environments adapted to being gnawed on, trampled by, and shit upon by mammoths still exist, I would have to argue with this notion.

Nature knows what it's doing when it comes to the environment including which species to create or destroy at any given time. Historically, it's humans that keep fucking up what nature never had a problem with.

Like yearly stabbing / crushing to death thousands of slow-growing, long-gestating, highly-social large herbivores, fragmenting their continuity while reducing their numbers and eventually rendering them extinct?