I am assuming -- but it's from years of experiencing it with current hardware or 3rd party software. Perhaps if we hired enough people we could go concentrate on those items to make that latest kernel work -- though that is not our core competency.
I suppose it is fear. Fear that IF we rolled our own and said, yes we will loose support for that 3rd party messaging software or clustering solution, but I'll take on support for that too if things break. Fear that the business is impacted by this, or fear that I could spend my days debugging 3rd party software. (Though really, Sr. management probably has more fear of this and would never allow it to go that far.)
If you mean that the 3rd party software could use a "thawed" kernel in an enterprise distribution -- I don't think we have any evidence (or hope) of tens or hundreds of them all agreeing to support their products on such a kernel. Inevitably it will need to be frozen.