The 51MP 5DS ($3,699) & 5DS R ($3,899) are high end studio and landscape cameras. Think medium format applications where large or finely detailed prints are required.
These cameras do not replace the 5D Mark III per Canon USA video.
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These cameras are positioned against the high megapixel full frame bodies like the 36MP Nikon D810 ($2,997) & 36MP Sony A7R ($2,098).
Canon USA mentioned medium format uses for the 5DS & 5DS R so here are the entry level medium format bodies by year.
2010 $4,500 40MP Pentax 645D
2014 $8,497 51.4MP Pentax 645Z
Now, why the drastic price difference? This all goes down to sensor size as shown below.
The Sony, Nikon and Canon uses a full frame sensor with a dimension of 36x24mm while the Pentax and other higher end medium format cameras tend to use a sensor with a dimension of 44x33mm or larger.
6X6 medium format film is included for comparison.