Who knows why that is. Maybe HN's audience has changed over the decades. Maybe your writing has. Maybe the novelty factor for Apple content is gone. Maybe there's just more competition for the front page now that HN is more mainstream. I just think it's unlikely that PG woke up one day and decided to screw you in particular.
The Simpsons had far too many seasons, but Matt Groening eventually went on to create Futurama. I hope you figure things out.
That “gradual decline” is an artifact of your maths, in which you're gradually changing the weight of recent years.
Consider a sequence with an extreme drop-off: 100, 100, 100, 100, 40. Taking averages of all the numbers, then all but the first, all but the first 2, and so on, yields: 88, 85, 80, 70, 40. That might look like it includes a gradual decline, but clearly there's nothing gradual in the underlying data.
The Simpsons had far too many seasons, but Matt Groening eventually went on to create Futurama. I hope you figure things out.