I have a similar problem. (although not as many miles)
I planned to take my family on a long haul trip, but the cost of the taxes is more than the cost of the cheapest economy ticket. And on top of that I cannot fly to the airport I want and possibly the dates I want are not available.

I think all the airlines do this tax thing, but AF/KLM are pushing the limits by pretending to charge taxes on their "free" flights when it seems they are just making up these figures as far as I can see. Certainly they don't offer a breakdown.

And since when did an airport have tax-raising powers?

IMHO its better to join A* or Skywards - one of those groupings that lets you swap your miles for aviator sunglasses or cheap jewellery.
500k miles could buy a lot of earrings.