It's not about "replacing" Mick...nobody is trying to replace anyone. The man got sick.
There are things to consider here:
- commitment to dates/contracts
- the salaries and livelihoods of 4-5 other people in Foreigner other than Mick
- fans who come to hear Foreigner songs
They can't just drop out of the tour to satisfy the "it's not Foreigner without Mick" minority...it's the music BUSINESS, remember?