I think that BGs do serve an important function in WoW. They add variety to PVP, and they offer an outlet for (population)balanced pvp encounters. While world pvp suffered in some ways from BGs, goodbye TM vs SS zergfest, a lot of other world pvp remained. World bosses became a conflict flashpoint. Blackrock mountain stayed a hotspot, and later Silithus/Plague lands became great zones for wpvp in their own right thanks to the additions of those raids.
So, I really don’t buy it that BGs killed wpvp. It only reduced one specific kind of world pvp that could easily be replicated by standing on the bridge in AV and aoeing. Or in Modern WoW, playing Ashran.
What really started chipping away at wpvp was the addition of flying in BC, and convenience teleports to instances. In TBC, the only real PVP hotspots I can think of were the areas where people were doing dailies, and Karazhan. There also weren’t many world objectives for players to fight over either, other than Nagrand for access to Elemental Plateau. The raids in Outland ceased being conflict points because people by and large would just flying mount bomb the instance portal with very little warning since the mounts moved so rapidly.