Dee did get a huge offer from ITV, and it flopped.
Quite a few people have left their "home" channel and had to either come scurrying back or faded to obscurity as the new format hasn't worked - including BBC to ITV or v/v swaps.
If the BBC can him, he'll be off presenting an almost identical show somewhere else - likely for a US network or Netflix - within months.
That's far more of a risk to the sales of Top Gear internationally than not having him on Top Gear is, as they will be able to find a replacement for that...