Hi Angelo,
I would be suspect about that PA5x... don't forget they've had youtube videos annoucing "coming soon" since 2018 (the PA4x came out in 2015, everyone assumed they'd launch Pa5x right after Genos). Not that it's not coming soon.. it's been 7 years, it probably *is* about time.
But a 61key Pa5x likely wouldn't make sense; the PA1000 *is* the 61 key version of the PA4x, and the PA900 was the 61 key of the PA3x (same sound library & chipsets, but different flash ram and physical shell); so I'd guess that when the PA5x finally arrives, they'll do the same (PA1100 would be the 61 key version).
An 88 key version of a PA5x would be intriguing though, as they offer no such 88 key equivalent (they do make an 88 key version of the PA300 though).
As Yamaha already has an 88 key instrument using the same sound engine as the Genos (the CVP809, albeit with a fantastic Bosendorfer in place of the C7, and no sample flash ram, and only 2 sound layers versus 3), it's inlikely Yamaha would ever do an 88 key portable version of a Tyros/Genos.
As for why the 76? All previous Tyros were 61 (the last time they offered the choice was the PSR9000, way back in 2000), and then they went with 61 key exclusively for a decade... at which point? the 76 key outsold the 61 key (at least here, anyway) about 3 or 4 to 1. They've been paring down options over the last decade (only 3 CVP models vs 5 for the previous generations, and in fact the current series only offers 2). It costs more to offer different versions, and for roughly the same amount of sales.
I'm not saying they won't ever offer 61 key flagship again, just that it's been more efficient for them not to. For now. Don't forget, by the time the next G2 launches? The PSRsx910 (or whatever they call it) will be for all intents and purposes a 61key Genos, just like the PSRsx900 is essentially a T5 (without the S.A2 sounds).
Mark
*May 28th correction: Turns out the C7 *is* in the CVP809! As a hidden voice. Those sneaky Yamaha engineers!