I think Paramount would've purchased them eventually, since they had distributed and marketed the first round of MCU films. And Viacom would be the envy of Tinseltown.
Or, if we're playing devil's advocate, WB could've purchased them. New Line was going to make their version of Iron Man and WB was close to getting the screen rights to Captain America -- so it had been close to happening. Had they realized the money to be made, they would have.
Warners would've had its cake and eat it too. Having Marvel Studios under its purview (provided they had Kevin Feige run it), as well as their own DC stable, would be a huge boon for them. They would be the ones booking 4-5 release dates for CBMs in 2023 and 2024, not Disney.