Pre-N64, really. I know this is a bit of a hot take, but the N64 started their downfall. Bizarre, nonsensical button layouts, an obsession with cartridges (which they tried to remedy with a failed disk peripheral) and other nonstandard game media formats, and just an overall obsession with using older tech than their competitors across multiple generations, that even still has persisted today in the Switch (though only in horsepower, really).

Their insistence on supporting primarily first party titles, with everything else taking a distant backseat, is mind boggling to me as well. And frankly, that's largely why their hardware has lagged. First party wise, Zelda and modern 3D Metroid are the only things to really challenge the hardware constraints, and it's easy enough to tell those teams to just deal with it. Everyone third party just has to down-port and crapify the graphical fidelity. There's a reason the eShop depends on 5 year+ old indie titles to still anything third party. If I didn't own those games on 2-3 other platforms already, I might be tempted.

Don't get me wrong, I know they're still successful. But I don't think they're good the way they were in the past. SNES and GBA were, IMO, the last truly great home and portable systems they made. Everything beyond that was contingent on gimmicks. I'm sure that's partly my nostalgia talking, but I do think it's telling that they're so dependent on remakes/ports/emulation of those eras and earlier.