Okay, let's think about this. A Mario Party game can take some while. Meanwhile, multiplayer games typically focus on keeping a group together for 15 or so minutes before having a "rest" period wherein people can drop in/drop out. Why? Because it's very likely that a lot of people will drop matches if they're forced to be in a game for a long period of time. Now imagine you're playing Mario Party online and one player drops mid-match. What happens? Does a CPU take over? If so, how difficult should it be? It ruins the flow of the game. Just look at online board games like Monopoly and how poorly the game reacts to the frequency in which people drop.
Here's the cold truth... Mario Party does not take well to being an online game playable with just anyone. A totally-online Mario Party would not be a fun experience given how long Mario Party games can be and the higher rate of randos dropping that will likely come with that. Rather, they should let you play full online MP games with friends. You and your buddies can plan out a game and since it'll be between friends, people aren't likely to drop randomly and screw up the game for everyone.
I think there is potential for Nintendo to add a pared-down version of a standard Mario Party board for random online play, and I think they'd do well to expand with more game modes optimized for online play. And playing full online MP matches with friends needs to be a thing. But I think Nintendo's at least made the right call not opening up full MP matches to be played with just anyone, not in their current incarnation at least.