I think you’ve got this bit wrong. I would imagine that most people would opt for the ad tier. By definition, if you don’t already have Netflix, you’re less likely to appreciate the value proposition, and so more likely to spend less rather than more.
And the more people on the ad-tier, the more valuable it is, both in terms of how many viewers are seeing ads and the data being collected to improve the product further.
But as @Lightning720 says, it’s not increasing their revenue from memberships as a result - but should longer term as their advertising plans pay off on the ad-tier.
ARPU figures that don’t include the advertising revenue are no longer useful for time comparatives as it’s always going to look worse now without giving the complete revenue picture.
It’s like Apple’s bullshit claim that they pay more per stream than Spotify. That’s just a function of maths because they don’t have an ad tier that monetises at a lower value but is still revenue adding to the bottom line.
They’re still made by the same people, so even if Channel 4 had kept doing it, you’d still have the same stories.
Brooker has also spoken of wanting to broaden what Black Mirror means, and get away from a narrow ‘weird and bleak’ niche. I think this is very sensible and if anything they should have done it sooner. Even in the first three seasons you could see where essentially the same story was being told again and again. Heck, even the very first Netflix episode was (to me) a retread of one of the best C4 ones.
Brooker took a break from making Black Mirror for precisely that reason, hence the gap between the most recent season and the one before.