Because they'd need to set up a complete multiplayer infrastructure to support it? How much would you consider it worth paying?
If they're planning MP of some kind for CP77, then I think it may be OK, and good, to use TW3 Arena as a test bed.
I was thinking the same exact thing. It'd be a great way for CDPR to try out and gain some multiplayer related experience, because it would be a reasonable first project.
I found Witcher 2's arena mode to be very addicting, and I believe it would be more intense and addicting if it was implemented as multiplayer. Playing against other players is more intriguing than playing against AI especially in an arena settings.
I never play Multiplayer myself, but I'd try it out at least to support CDPR.
I think 10-15 dollars would be reasonable, especially if there were different arenas (maps) and challenges (duels, team vs team, everyone killing each other, and so on)
But I fret the public's reaction would be negative, because they'd start accusing CDPR of creating paid DLC that should have come with the game.