Well, for one, it's Wenger's fault that they wanted to leave in the first place. You got 44m for the pair, not 50, and Cazorla was 15-16 was he not? There's a 10m disparity already corrected.
In any case,
football is not about building a bank balance.
A 20m player on the pitch is far better than 25m in the bank balance. No doubt they were good transfer fees but have they been replaced by players of 24m and 20m worth of quality? Nasri maybe, but Cazorla is still an inferior (only a little worse, they're on the same plane I guess) and less successful player, and van Persie has not been replaced properly at all. What's the point of a super rich club like Arsenal getting even more money if they can't use that money properly, what's the point of a 24m transfer fee if only 9.6m of it is going to be spent, that too on outright mediocrity?
Selling van Persie and Nasri can not be seen as a success for Arsene given their roles in their new clubs' significant successes whilst Arsenal replace them with inferior players and continue to fail on the pitch compared to the bank balance. He got good (not incredible, just good) fees for them but he failed to use them properly, and that too at a club where there's plenty of money. This £160m hasn't accumulated overnight and there's no doubt that money could have been spent and Nasri/van Persie kept simultaneously.
As for Wenger's overall transfer record, it's largely very good, not so much recently. The Anelka deal is amongst the best in the history of football.