The fiat collapse literally will happen even if Bitcoin's code base vanishes from the earth. The collapse of Fiat currencies in their current use is not the only fiat death spiral in history. Again, study economics. Bitcoin is simply an alternative like gold but without all of the flaws physicality brings (mobility, transactibility, and confiscation risk).

The point you are missing is bitcoin doesn't HAVE to succeed for the dollar to die. Bitcoin is just the lifeboat for that sinking ship. People holding, using, and trading in an alternative is what makes Bitcoin replace the dollar. The dollar collapsing is the inevitability. Political and institutional adoption of bitcoin will just make the collapse WAAAY less severe(for bitcoiners).