The wordplay is more than nice, nevertheless, I’m not sure whether it is a good slogan, but more for the reason that it is too obvious… Personally, I like understatement much more than aggressive “self-presenting”, and - as I’m a scientist - facts (if available) are (mostly) a better foundation for decisions than emotions. I think that is similar to what you wrote…
On the other hand I had to learn - and it was hard, believe me - that also scientists are humans with convictions, believes, biases, and their conclusions and decisions are quite often (in my field > 90%) obviously based on emotions, and therefore may be just the opposite of what the facts suggest. This means: emotions are decisive…
And I’m quite sure - you will not find many people who would support the idea that products are sold just by facts…
Even the product name “Fairphone” is serving the emotional aspect, it is the necessary vehicle for the transportation of facts. The facts are much too complicated, look at the suggestions. Sure, it is an interesting idea to build up the name Fairphone of some parts which can be easily exchanged: but the message can’t be recognized at a glance, it needs to be explained - and at that moment you have lost. At best, it is overlooked, at worst it results in negative associations: how complicated! A new gadget for nerds. Not cool.
Furthermore, your argumentation would be equally valid concerning the name of the product itself - “Fairphone”. Even if your argument is considered true (difference to other phones too small) it is somehow inconsistent…
And the slogan does not say that the Fairphone is a fair phone. It just says that other phones are not fair. And that is definitely true, no?
In conclusion, I have the feeling that the discussion on this slogan should not end with your statement.
Basically, it has everything a slogan should have to wake interest. And nothing else is the idea behind a slogan. It is easily understood, it has an intellectual touch (word play), it transports one of the most positive aspects in human relationship - fairness. And it transports the message with a twinkling eye, with humour.
Well, the first version of my post started with “I’m basically critical towards this slogan…”, and the actual version also starts still critically. But while writing I changed my mind. I did not rewrite my post completely to show the development.
In the end, if the Fairphone is not successful - and there are some very strong reasons why this could happen - we lose altogether. Not only some money for a phone with exchangeable parts, which may or may not be produced in a better quality in future (thus ending with buying one of the other phones which are definitely not fair [btw: can a product be “fair” at all, isn’t that an attitude or behaviour of thinking living beings?]), but - much more important - we lose also the idea that fair production is not only nice but can also be successful.
Thus, I think it is important that the Fairphone is successful, and in my opinion this slogan promises to be successful to draw attention.
Sorry for my long post…