Quite true, but I doubt there is a way for this merger to happen without YX miles being devalued, simply because YX has actual partners and is a better overall FF program. Unless FL becomes really scared of losing YX passengers and gives a very generous conversion.
My conversion guess was 1500 because:
OW YX award = 15000 miles = 8 FL credits (1875 to 1 ratio)
RT YX award = 25000 miles = 16 FL credits (1562.5 to 1 ratio)
At those levels, 1500 is a bonus.
12500 bonus miles for YX top credit card = 6 credits for FL top credit card = 2083 to 1 ratio
10000 bonus miles for YX CC BT = 10 credits for FL CC BT = 1000 to 1 ratio
5000 bonus miles for YX bottom CC = 3 credits for FL bottom CC = 1666 to 1 ratio
And of course, the CCs are issued by the same bank. Hopefully they'll pick the 1000 to 1 ratio, at which point the Asia and Europe coach awards will still be devalued, just not as much. I can't see a ratio below 1000. For RT coach awards it would be a 50% bonus.