Maybe not 'crucial', but definitely helpful and important.
Also, it's a bit much suggesting that it's flat design that's what's responsible for smartphone uptake. If that was the case, then hell, it was skeuomorphism that exploded the market in the first place.
Also, it's a bit much suggesting that it's flat design that's what's responsible for smartphone uptake. If that was the case, then hell, it was skeuomorphism that exploded the market in the first place.