Sony's overall install base will almost certainly have dropped quite a lot too (probably from around 170m~ to 110 - 120m). It's a much less dramatic drop than Nintendo's, but it's still worth pointing out. The shift from the 7th to 8th gen hit the handheld market like a ton of bricks. The 3DS should be able to keep selling decent numbers for a while longer though, so the gap will be slightly small than it is now.
Anyway, the Switch selling 80m would be great for Nintendo. The 8th gen was difficult for them because it involved 1 system that effectively failed, and another that required losses to get it off the ground. They were also likley expecting both to perform better than they did, so were caught off guard. Right now Nintendo's biggest focus is putting the losses/relatively small profits of the last few years behind them and finding some stability in a market that's desires have shifted from what they used to offer. The Switch selling 80m might technically be a decline, but it'd fulfil both of those roles well. Given how conservative their Switch estimates are for FY2017, i wouldn't be surprised if they've planned for the Switch to fulfil those roles with a lot less than 80m.
