It's not when your target is AGA+68020/30 and possibly A1200+trapdoor fastmem. Why do we have to take higher end systems in account, and make sacrifices on the low end, while the software could work perfectly fine on low end?
This is Amiga we're talking about, not some high end computer where making use of the OS for everything might make sense. For low end (68020/30) using the OS is fine. Well behaved software is nice, after all, and killing the OS (except for WHDLoad and demos) isn't a good thing, same for not using the OS's screen open functions. But blitting has to be fast on low end machines, or the software is going to run like crap.
Why does everything have to be adapted for high end machines and custom expansion boards? Want to run Amiga software? Use an Amiga.
With the blitter, yes, but the blitter is too slow.
It's also the only way to get good performance on low end machines. It's old fashioned because the hardware is old, and many Amiga users use this hardware.
It's not like we can't detect the machine we are running on and use a different codepath for each architecture. Best of both worlds.