The Legacy play makes sense. It’s a one-of in Legacy Sneak Attack sideboards, since it is technically possible to cast Emrakul, the Aeons Torn and take advantage of Kozilek’s Return’s second effect. But most of the time this is a Pyroclasm for one more mana that also happens to be colorless - meaning it dodges things like protection from red or protection from colored spells, which can be relevant at times. We’ve also seen a side-boarded copy of Kozilek’s Return in the occasional Miracle Control or OmniTell deck.
The Modern play is a bit more confusing. Primarily, Kozilek’s Return has been seen in Grixis Death’s Shadow Aggro decks. Again, it’s basically a colorless Pyroclasm for one more mana, as there is no Eldrazi to cast in that deck. But, yet again, being colorless helps against certain creatures, such as Burrenton Forge-Tender, which sees some sideboard play.
As a one-of in Modern and Legacy sideboards, and only barely any Standard play towards the end of its Standard life, it’s hard to see this card going much lower in price. This is definitely a good card in certain Eldrazi decks to help clear the way for the Titans such as Ulamog and Emrakul. Its exile ability is certainly decent, too. Of course, the main thing helping it keep its value is that it’s a mythic rare. Plus, being an Eldrazi-themed card, its price is propped up a bit, as well.
Towards the end of its Standard life, Kozilek's Return often listed for $5, but no one paid more than $4. As Oath of the Gatewatch stock was liquidated around October 2017, it was $2.50 or less. In 2018, it was barely selling for $1, despite a much higher median price. But it's worth holding onto if you already have copies. There is a chance that this card gets adopted by more Modern and Legacy decks in the future.