How would you have felt if Ultima Online had been released under the title Ultima 9?

In this case, Ultima 9 would have been Ultima 10, and the cancelled Ultima 10 would of course have been Ultima 11?

(I note that there are a couple well-known JRPG series that did just this, though the numbers may be different.)

I'd have felt nothing, since Ultima 9 is dead to me. Bugs aside, there are so many lore-breaking atrocities in that game.
In my mind, when Ultima 8 finishes and the Avatar returns to Britannia with all his newfound powers, he destroys the Guardian in a spectacular 1 on 1 battle and that's the end of it.

I've never played Ultima Online, since around that time I did not have a decent internet connection.

How would you have felt at the time, given that you hadn't experienced the game called Ultima 9 (and there was no indication that it would turn out poorly), and you saw that the game being released under the title "Ultima 9" would not have been a game you could play?

It would have depended on if this new Ultima game would have been part of the Guardian story line. If the story would suddenly have been continued in a multiplayer game, or different platform, I'd be pretty mad. At least I still have the choice to play Ultima 9 and finish the Guardian story line, if I'd want to torture myself through that.

There's this game called Drakan: Order of the Flame, and the game very clearly ends on a "To be continued", a huge cliffhanger. Unfortunately, the sequel never came out for PC, only PS2, so I was unable to ever finish it. It sure pissed me off.

I don't mind missing out on single-story games, but when a story is told over multiple games and I can't play all of them, then yeah, I get pretty salty about it. It's like watching the first two Lord of the Rings movies, and never get to see the third one.

Or there's cases like The Magic of Scheherezade, or Final Fantasy Mystic Quest, where the game ends with the main characters leaving on another journey, implying that there'd be a sequel, but the sequel never came. (Apparently, there was a sequel to MoS planned, but it was never released, which is a shame because of how that game was both unique (Zelda-like, except that once in a while you get into a turn-based battle in addition to the usual action combat), and ahead of its time (combination attacks, which otherwise weren't seen until Chrono Trigger and Phantasy Star 4).

Or a case like Paladin's Quest, where there is a sequel that follows the story, except that the sequel (Lennus 2) would only see a release in Japan, and the third game in the series (which the ending strongly hints at, including the planet it would take place on) was never made.

(For comparison, imagine if Return of the King didn't exist, whether as a book or a movie, but the first two books did still exist.)

As someone who played Ultima Online off and on since it was new, I would have been rather confused. Mostly because there was no larger story to it (to a degree that has changed but the story is more to explain changes to the setting, like the fact at this point Lord British is completely gone and Blackthorn is officially in charge), and you are playing some random person, rather than anyone connected to the Avatar. And of course UO ignores after everything Ultima 1, using the destruction of the Gem of Immorality as the in game justification for the various servers (though Minax later was involved in one of their occasional bits of story).