It seems a little dramatic to think Nvidia would spend $40 billion just to run ARM into the ground.
In this case the ROI does not depend on nvidia able to leverage ARM in their portfolio, even if they stopped developing ARM now , the licensing alone is worth a lot, while they hope to do more than that they don't have to.
IP can be extremely valuable in a merger like this , Google bought motorola and Microsoft bought nokia for largely IP reasons at 15B and 8 B