a big company don't own the .co extension but they have the .com
What are the chances it will acquire a long .com related to their company ?
I assume you got mixed up with the .CO and .COM in the first sentence, and if so, this comes back to the old "Reign in Hell / Serve in Heaven" question - would you rather have your core brand in an alt-TLD like .CO, .IO, etc. or fall back to a longer domain in order to use .COM.
The answer really depends on the company name and brand. Single-words are far more valuable and useful in alt-TLDs, and if a company called Dolphin Solutions already has something core like Dolphin. io, and the .COM is in use, there is no reason to change over to DolphinSolutions. com just to be on a .COM. It's potential brand protection as a forward, but again, it depends on the price.
Other examples are a bit more murky, but it really comes down to how much of a brand match the company already owns in their non-COM, how much longer the .COM would be, whether the core brand in .COM is in use or available for $$$$$, the price of the long-COM, and about a zillion other factors.