In case of the HD 4770, GDDR5 memory with 128-bit memory bus will provide identical bandwidth as GDDR3 with 256-bit bus, provided they both run at the same speeds.

GDDR6 is essentially GDDR5 ported to DDR4 cells. So GDDR5 stuck around for so long because DDR3 did. GDDR3 (the GDDR version of DDR2) did similar, and as mpat points out, had a similar lifespan.

Unlike in FarCry 2, here we see that HD 4850 GS GLH 512MB GDDR5 is constantly beating the HD 4850 GS 1024MB GDDR3.