FB is a pretty lousy programme for mileage earning in economy class. You are right.

I used to earn far more FB miles from spending on my Amex FB card rather than by flying - but then I decided that there were better rewards for my credit card spend and I ditched that card years ago.

But the truth is, there are very few generous programmes for flyers in Economy class. Programmes like BA's Avios are typically more generous, and there are usually far more ways to earn Avios (shopping, petrol, etc) so depending on where you live, you might find your mileage-earning needs are better served in a different programme (though, of course, if you can only fly AFKL then Avios are no use to you!).

However, the one good thing about Flying Blue is that you can earn status quite easily and cheaply just on the number of segments. Having lounge access and/or the Skypriority benefits are really the only thing to get excited about in Flying Blue. FB is really NOT the programme to be getting exctied about as regards ammassing Award Miles.

Ignoring status bonuses, and assuming the traveller remains without status throughout, you would have to complete 23 return AMS-LHR trips on KLM in order to have the absolute minimum mileage earning for a single one-way AMS-LHR trip. On BA, you would have enough after 16 return AMS-LHR trips (and up to a year ago the earning on the cheapest fares was higher, so it took a lot less). BA also allows you to make part-Avios, part-cash redemptions, so you could redeem an award for the outlay of some extra cash much sooner.