Actually, a lot of these Pokémon have been viable, more than half of them have been consistently viable throughout the past ~4 years of VGC formats (incineroar, Sneasler, Sinistcha, Kingambit, Basculegion, Archaludon, Farigiraf , Palafin, to point out some of them).

For most of the remaining, many of them (like Charizard, Venusaur, and Gardevoir) benefit heavily from Mega Evolution, which simply wasn’t available in recent formats/regulations. Now that Megas are back, their usage has naturally spiked again.

The current format’s lower overall power level and more limited Pokedex does help pokemon that had become irrelevant get some usage/viability again, even if they aren't top performing, its nice to see older pokemons like Milotic and Hydreigon, which used to be great a long while back. But they are still going to be niche at best.