It'd be acceptable to keep making them if they didn't keep coming at the rate they do. Mario Kart comes one per console. Heck, Mario PLATFORMERS and RPGs come one per console. Parties came three or four. And for a while it was okay, because there were real, genuine improvements that made the new one worth playing. But if they weren't made as quickly as McDonald's burgers and each one was made to BE the one of the console, utilizing the system's features to their extent, think of how much better they'd be. We'd still be at Mario Party 3 with the Millennium Star hosting Wi-Fi.
But I don't know if Wi-Fi would motivate me to buy the next one. I have lag, and I doubt anyone here would rather play Mario Party 9 than Brawl or Mario Kart anyway. I sure wouldn't.