I played WoW from day 1, until the end of Firelands. Just to qualify my opinion as not completely bs, but only just partially instead :P
Later arena, but first BG's started to push people into instanced, reward-based pvp and killed world pvp. That's completely true. However, the game probably wouldn't have thrived on Southshore-Tarren Mill battles quite as well as it did on BG's and arenas. While instanced pvp was bad for nostalgia's sake, it was good for the growth of the game, as the subscription numbers would indicate.
People who don't like the accountability of arenas but do like to pvp often complain about how arenas are killing bg's. People who hate the large team size and diminished personal impact of bg's but like to pvp often criticize bg's for not highlighting personal skill the way arenas do. People who like pvp but don't like the accountability and tight coordination of arena, but also don't like the loose coordination of bg's tend to criticize both for having killed world pvp. In the end it's all very cyclic.