keep him. earn another one by getting the wins. also once the event finishes and there's only a finite number of Arrietas out there, the price will inevitably rise and he'll end up being another Buy Now 0 / Sell Now 1m card due to a lack of available cards. I'd hang onto it unless you really need the stubs (in which case I doubt you dropped 625k on him in the first place).
I bought the Prime Britton for 200,000 and just sold for around 282,000. As the cards go back into circulation, buy them, then hang onto them knowing they're going to become more hard to find as time goes on. Looks at cards like Will Clark (Giants) from previous events. Hovered around 30,000 for a while. Last time I looked it was up to about 75,000 and no way to get him other than buying off the market.