If you remember, they stopped doing 'series' of Morse's some time before Morse came to an end. (A series usually being 3 x 2hour films)
They then kept coming back and doing one offs as Colin Dexter produced new Morse books. Finishing of course with The Remorseful Day. Of course that doesnt apply to Lewis as each story is original to tv.
To be honest, a lot will be down the actors. If Lawrence Fox does well in the USA, then you wont see him again. I suspect he sees Lewis as a stepping stone to greater things.
If on the other hand Fox doesn't pull it off in the USA, and not all actors do - then I would have thought a few years down the line it is not impossble he would come back to Lewis.
I would have thought Kevin Whatley has less options. No disrespect to the guy, but he is soon going to be too old for leading man material. So being in a series where your character, Lewis is actually in the title is no bad thing. I don't see Kevin Whatley being in anything as big again. It will be guest staring parts like he was in George Gently last year.
So I would not be surprised to see him carry on in Lewis without Fox. He might take a few years off and then see that being in the odd Lewis is no bad thing when you are an actor the wrong side of 60 years old. ( and I mean that with no disrespect to the guy)