I've been toying around with the audio mixer to see if I can get some intuition on why the linear blending of samples doesn't make much difference. It turns out that it actually does make a small but perceivable difference in some cases. If you take day 146's source code, silence the piano music and play "bloop_01.wav" at dsample=0.8 with no sample blending, you will hear an extra low-volume, high-pitched electric ringing along with the original audio that is not present when interpolation is used.
To illustrate the origin of the high-pitched component I will be using a 440 Hz pure sine wave instead of the more complex bloop sound. If you play that wave through the HMH mixer at 80% of its original speed (dsample=0.80), with and without linear blending, grab the outputs and superimpose them, you get the top plot in the following image (x axis: sample number, y axis: volume(-32768, +32767), green: blended, blue: nearest sample). The bottom plot shows the difference between the two waves.
As we've already seen in the HMH stream, mixing two sounds can be achieved by adding them. By that same logic, the non-blended sound can be seen as the combination of the blended sound plus the sawtooth wave at the bottom (note the change of vertical scale, though), which is the high-pitched sound we were looking for.
The frequency of the sawtooth wave comes from the aliasing generated by sampling every 0.8 samples. We're taking samples at positions 0, 0.8, 1.6, 2.4, 3.2, 4.0 ..., which means we're hitting one original sample out of every 5. With a sampling frequency of 48000Hz, that amounts to oscillating at 48000/5 = 9600 Hz.
Since the frequency of the input sound is much lower than the 48kHz sampling frequency, the volume of the sawtooth wave wave is mainly determined by the 16-bit quantization of the source file, so it's bound to be low.
The original 440Hz wave lacks a 9600 Hz component, which makes the sawtooth wave easy to spot. However, in a less structured, more frequency-rich sound environment, this extra component is likely to end up masked beyond perceptibility. That is not the case, however, when we only play the bloop sound, whose frequency spectrum is silent in the 5kHz-11kHz range.