My late FIL had a direct cremation, my DH really struggled with processing it. Yes, you can organise something, but it's really not the same (IMO).
I think it's very important to respect what the deceased person wanted; but I'm not convinced that the direct cremation industry necessarily does its due diligence as to whether it is what their customers truly want, or whether they just see it as deal done and happily count the money. I highly, highly doubt - with this one-time purchase - that they would ever follow it up periodically to check that you haven't changed your mind.
I think that, as well as those who go into it with their eyes open and do definitely want it, there will be a lot of people who are swayed by the advertising, the idea of saving money (and leaving a bigger inheritance) and 'not wanting to be any fuss' - who might make a one-time decision that, were they able to fully consider the ramifications in retrospect, they might well not have chosen.
I think it's a shame, in a way, that it's obviously impossible to be able to make your own funeral plans at the time when you would theoretically be in the very best position to do so: after your death. It's a very 'British' thing to do to feel awkward at thinking about yourself being the centre of (and purpose) of a celebration - which I wonder if the direct cremation companies are capitalising on.
I also agree that, whilst it's important to respect people's genuine wishes, the funeral is mainly for those left behind - so I personally wouldn't want to choose the funeral that I would want at the expense of what might be most meaningful and helpful for my loved ones.