If you go back far enough (or look at a legacy carrier) duty pay was an "extra", often taxed at a much lower rate than salary (if at all) & was therefore a very nice little fillup to a probably more than adequate basic salary.
Fast forward to the brave new world of Ryanair etc, where the basic salary is relatively small, or in the case of 3/4 of their employees (sorry "service providers" ) non existant. So no fly= no pay / sick= VERY sick !
For me, I think we should all be paid the same, & thereby provide an incentive for crewing/rostering to do their job properly and use us efficiently.
The current system is also abused in some companies by those who manipulate it to have all the longest sectors etc, thereby creating extra work for a crewing Dept.
Far simpler to say "I am available, use me/abuse me as much as you legally can & if I have a quiet month it's your fault but why should I pay for it".
That way you also have an idea how much you will earn each month, rather than a system where it can vary between b@gger all & 10,000€ at random, depending on month of the year/sickness/ the ineptness of a Commercial Dept or Crewing Dept.