There is probably a whole backstory that we're not aware of.
My theory is that the tour was already signed while Mustaine was unaware of his condition. They could've been planning to shoot the new album in summer '19 so that European tour would make sense. Since it all got a bit delayed (if I concluded correctly he missed Megacruise which means he is still probably under treatment).
Maybe Mustaine though it would be good idea to still tour, but with less burden, so they change the headliner.
Maybe he wants to invest in something, hence he didn't want to delay his paychecks (he did sold majority of his equipment recently).
Whatever it is, it a sad thing to see. Maybe they could organize it a bit differently. Hiatus until the new album, release it late spring 2020, hit the bigger festivals in Europe avoiding headlining slots and then proper USA headliner tour.
I mean, only reason I've heard of 5FDP is because one of their guys had a mental breakdown on stage and Bad Wolves are announced as "that band who did great cover of..." I mean, what the f*** is this, the 60's?