Look at it the other way around: if you know that you’ll be usine miles rather than pay for a trip you want to make, then by all means get miles on your account. If you don‘t get them through flying, then you can buy them. And then, and only then, this 100% bonus is a „good“ deal.

I have bought miles in the past, simply to pay with miles flights where a cash payment would have been much more expensive. For instance, for buying one-way tickets for a family member to the US or to Brazil. Or because I was getting someone in Premiere from Paris to California and back from East Coast, and the return award was 440,000 miles and the ticket would be more than 11,000 EUR - in that case, buying the required miles for 6,200 EUR is a good deal.

So don‘t think „I can get miles cheaply“ but think „I want to do a certain trip > what is better miles or cash > if miles, how much does it cost me in money and time to fly to get those miles > buy if that is more advantageous“.

And one other word of caution, when you make your calculation, take into account the fees/taxes that the airline will charge you even for award tickets. If you find a return in Premiere for 440,000 that‘s great, but you may end up paying 600 EUR on top just for taxes. In the above example it‘s still less money to buy the miles at a discount and pay charges than to pay for the ticket in cash. But the difference becomes smaller.

PS: mathematically, it is less expensive to buy miles than to get them from flying. Take my above example, you want 440,000 miles for that once in a lifetime award ticket to New York. You can either pay 10,000 EUR for the ticket, or do mileage runs where even as a Platinum you would need to spend 55,000 EUR to get 440,000 miles, or spend 6,200 EUR to buy the miles under the current promotion. If you do mileage runs only to get the miles but otherwise would not make those trips and you‘re rather annoyed by the time investment to make mileage runs - then buy. But only then! Because obviously the 55,000 EUR spent on tickets that get you 440,000 miles can actually get you somewhere very nice - when it‘s not a mileage run then the purpose of the trip is to do the trip and miles are an additional benefit, not the sole objective of the trip.