A card is replaced by a newer model that fits in the same price/performance range and is cheaper for ATI to produce.
A card that's more than twice as fast as a 4870 (5870) and twice the price should not be considered a replacement, but rather an upgrade.
The new generation card (58xx), replaces the last generation card (48xx). Why wouldn't they be better and faster, it's the next advancement in technology? If you have your mindset, where is the replacement for the ATI 9800? The 4870 was $300 when it first came out, as the 5870 will not be $400 in a years time.
The Nvidia model numbers are not so easy to figure out, as they got away from the 4 number sheme. If you go by 3 generations ago, the 7600GT was replaced by the 8600GT, as the 7800GT was replaced by the 8800GT. Granted there are all sorts of other models thrown in there that were never continues and that were not direct replacements. Teh 7950GT was never given a direct replacement, they consolidate models and such. They had a 7800,7900, and 7950GT card, but the 8800GT sorta replaced them all. Granted they later came out with a 9800GT, but it was just are die shrink of the 8800GT. That was a "replacement" as well, but not a next generation replacement.
We could argue about this all day. but the 4870 was NOT replaced by the 5770.