A lot of vtubers just don't realize why some things are done routinely across numerous corps in the industry. Sometimes it's just Japanese habitual autism and other times it's actually a proven strategy. For instance, a while back Lumi stopped including which games she was planning on playing in her schedules she'd post. I assume this was done because she didn't want people to see the schedule and go "ew, I don't want to watch that stream because she's playing that game" to bait people to show up anyways and become regular watchers.
I fall into that category of viewer. I'd show up for the streams that I was interested in and if she swapped to Genshin, I'd just close the stream. Not including the games she's planning to play in the schedule isn't going to magically make someone like me show up for every stream and then decide to stick around when I find out that she's playing Genshin. That change had the opposite effect for me and I've basically stopped watching Lumi altogether despite her being one of my preferred streamers because I have no idea what she's streaming. Meanwhile I have added streamers to my preferred list purely because I saw them post they were going to play a game and I wanted to check out their stream because of it. There is a distinct reason why stream schedules have the stream topics listed and why waiting rooms are supposed to be posted in advance.
The type of person that looks at a stream schedule and thinks "wow, I won't watch these 2 out of 5 days because I don't like those 2 streams" will at best show up to those 2 days and then close your stream because if they weren't going to show up because you posted it, they're not going to stick around when they find out what you're streaming. However you have no longer given them an incentive to remember to check out your stream for those 3 days for the streams they are interested in.
Schedules set me off, but for different reasons.
The vast majority of Hololive JP don't use schedules. Past the first few weeks after debut, it's just not a thing, they put up stream reservations instead and that's good enough. There are some exceptions but they are just that, exceptions. But even so, schedules are the one little bit of EN autism that has filtered all the way back into the JP indie sphere, and I am going to point the blame at the most autistic generation in Hololive history: Hololive English Myth. Of course they didn't invent schedules, but they sure made them a thing for the entire vtuber world. This is the gen containing the rap autist, the hikikomori, and the german. Managed by the kind of watch wearing busywoman who eats breakfast by mixing all the food from the buffet into a single bowl because it's more efficient. Myth had shared calendars since pre-debut so of course they have schedules. And so now all vtubers except HoloJP and NijiJP do schedules, even though flesh streamers usually don't and the biggest JP vtubers in the world get away without just fine.
Anyway, I do think Twitch is a different beast to YouTube in terms of viewer behavior and structure. Twitch viewers will absolutely jump ship when a streamer switches categories for a game, while YT viewers are far more sticky going from chatting to gaming (in all other respects Twitch viewers are stickier than YT).
There are structural reasons too. On both Twitch and YT Gaming you can tag what game you are playing, but Twitch lets you change categories mid stream, while YT you can only tag one game per video, and typically do so when putting up the waiting room. That's why it's usually new game = new stream for JP corpos these days, even for marathon streams. Not to mention discoverability on YT depends on thumbnail, which has to be created in advance, so if you need to know what you're doing in advance anyway, may as well make a schedule.
All that together, schedules are necessary because Myth trained the viewer and all their indie admirers to expect them, and in part because YT streaming tends towards pre-planning your stream with thumbnails and pre-tagging. The oldschool Twitch vtubers who predate Myth, and anyone mostly in their influence, often do without, because the Twitch audience tends to care less. Not sure about your personal experience with Lumi but because of the reasons above, schedules are far more of a vtuber audience expectation than a general viewer expectation.