They dropped them only much later, in Coldfire, not in 68030, which was completely compatible with 68020. MC68040 dropped some features, then MC68060 dropped more features, but for user programs they remained binary compatible with MC68020.
They have probably realized quickly that adding memory indirect addressing modes was a mistake, but they could not drop them because that would have broken all programs ported to MC68020.
With Coldfire, they were able to prune the unwanted addressing modes, because Coldfire was intended for embedded computers, where the software is recompiled for every new project and binary compatibility with legacy programs is not necessary.
Both the addressing modes of Intel 80386 (1985) and of Motorola MC68020 (1984) were inspired by the addressing modes of DEC VAX, but in a very rare event in the history of Intel processors the Intel designers have done the right thing by choosing the useful subset of the DEC VAX addressing modes, while the Motorola designers have made a mistake by choosing to implement an even more complex set of addressing modes than that of DEC VAX.
The 68030 really only dropped the Call Module and Return from Module instructions, which nobody used anyway since relatively few developers wanted to write code only for 68020 and higher around the time the 68020 and then 68030 shipped.
They have probably realized quickly that adding memory indirect addressing modes was a mistake, but they could not drop them because that would have broken all programs ported to MC68020.
With Coldfire, they were able to prune the unwanted addressing modes, because Coldfire was intended for embedded computers, where the software is recompiled for every new project and binary compatibility with legacy programs is not necessary.
Both the addressing modes of Intel 80386 (1985) and of Motorola MC68020 (1984) were inspired by the addressing modes of DEC VAX, but in a very rare event in the history of Intel processors the Intel designers have done the right thing by choosing the useful subset of the DEC VAX addressing modes, while the Motorola designers have made a mistake by choosing to implement an even more complex set of addressing modes than that of DEC VAX.