No one will be a hostage to their airline because of an ATPL flight test requirement, the seniority list will be enough of a reason to keep someone in a seat.
Well yes they are. What about someone in their late 30's at the bottom of huge seniority list realising they will be late 50's before they have the opportunity for a command. They want to go overseas so at least they will see a command, where are they going with an Australian CPL? If you don't have a US or EU work Visa you are limited to Asia or the Middle East, all of which require ATPL.
The thing in Europe is that it doesn't take forever to get a command. The likes of RyanAir and easyjet don't have 20 years to a command