That's how I feel about STEP you. This is so far the only recent ayu songs that I would think it could be a million sellers if it was released back in her peak era.
Of course I know it's not a one hit wonder

. But I kinda feel it's sorta like somehow. Yes SMILY/Planetarium were really popular as well, but it's not up to par with Sakuranbo in terms of longevity and sales (even tho I hate this song). And too bad it seems like Ai-chan cannot make another 500K seller (judging by her recent sales..). It's like, if you ask anyone which ai-chan song is the most popular, everyone will point towards Sakuranbo.
Ayu or hikki are different on the other hand. They have more than a few songs that hit million marks (ayu: B&G, A, SEASONS, M, or even evolution, which was about 960K sold) (hikki: Automatic, Addicted To You, W&S, CYKAS?).
About ayu's most popular songs being released in her prime theory.. Well I sorta disagree. As a fan going through LOVEppears to GUILTY era (well too bad I wasn't into JPop when she released A Song for XX), I still consider SEASONS, M, evolution, etc. are one of her best works. Those songs are more unique in their own ways, unlike recent her works. Honestly I cannot see the difference between fairyland/BLUE BIRD/glitter. They have the same formula.. similar melodies, similar tones, etc. It's like whenever she's releasing a new single, it just sounds like recycling her old materials. That was how I felt back in I am to RAINBOW era (e.g., Free & Easy sounds too similar to M). And thank god she released Memorial address, in which the materials sounded quite different compared to the works in I am and RAINBOW. And somehow I believe ayu's going through the cycle again, as I have faith that she'll be releasing some spectacular like Memorial address again this year.