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I think in the end the problem for Amiga graphcs was lack of appreciation for using page mode for DRAM accesses.
Sure, fmode came later, but it was still limited.
The key to maximizing bandwidth for graphics (and CPU, for that matter) is exploiting DRAM's ability to produce after a RAS an entire page of bits to be accessed with CAS.
Amiga's interleved DMA is great for random accesses by mutiple devices, but for sequetial accesses it just isn't the best idea.
Exploiting page mode is what helped VGA take off.
DRAM used in the a500 had a best case random access time of 260ns. (The Amiga DMA ran them with a 280ns cycle.)
But consider using page mode for the very same memory in 1987. After the initial RAS and first CAS, each additional access to the page takes just 75ns -- more than three times as fast.
The Amiga did get fmode later, but it was still limited. The page size for 41256 parts in the a500 was 512 bits. With 16 chips that gives 1k bytes per page. VGA cards could exploit this for the highest resolutions and bit depths.
What's interesting is just how little improvement there's been to random access times for DRAM in the past 30+years. A random DMA in 1985 for the Amiga was 280 ns. For today's CPUs and GPUs a completely random access takes about 60ns. The huge gains in memory bandwith you see today are mostly due to accessing the same page as quickly as possible and filling caches once a row is opened.
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