Battle Spot is more competitive precisely because the teams you face there have already had extensive PS testing or at least a lot of painstaking theorycrafting behind them, else the player wouldn't have wasted their team breeding the team. PS is still a valuable resource since it's much easier to make minor tweaks over time and play a larger volume of battles.
Come next season, (English) VGC players will probably migrate more to playing on PS exclusively once the Battle Spot rules are no longer equivalent to VGC rules.