Regress is a strange card to me. There have been many blue bounce cards over the years, from the classic Boomerang to its Arcane version, Eye of Nowhere, to utility bounce spells like Unsummon. Most cost one or two blue mana. But here we have Regress, functionally identical to Boomerang, but costing 2U. Were people not playing Boomerang because of the UU cost? I honestly don’t remember anyone ever playing this card in either limited or constructed back when Mirrodin was current. Of course, if you were playing blue in Mirrodin block you had MUCH better cards to play: Broodstar, Thoughtcast, and Thirst for Knowledge were staples (later Broodstar fell out of favor). With all the Affinity goodies blue had at its disposal I can’t imagine why they’d bother with Regress. And they didn’t.
Thornbite Staff was part of the cycle of tribe-based equipment. Sadly for Thornbite Staff, Shaman as a tribe never really took off. This ability is interesting and can be powerful but at 4 to equip and 2 to activate it is much worse than the card it is trying to turn your creature into, Goblin Sharpshooter. Of course, the tribal theme should allow you to avoid paying the equip cost but the two to activate is still rough. In order to truly break the card you would need a lot of mana and some way to sacrifice a lot of creatures. Sure you can kill a bunch of x/1’s on the other side of the table but the true power of the Sharpshooter is to win games out of nowhere. Other goblins like Goblin Sledder and Skirk Prospector let you sacrifice a bunch of creatures for free and potentially kill an opponent without even bothering to attack. Thornbite Staff’s activation cost prevents such trickery, making it a more balanced but much less fun card.