I'll shoot that right back at you- THINK for a moment about how inoften a rushed ending is NOT the result of a story being axed and instead simply being poorly written. Usually those shitty stories end up getting axed (oh gee!) before they even reach the ending, or the author, now better than they were when they wrote the original WN, can fix the ending pacing for the adaptation, which also happens a lot of the time! It's only a vanishingly few that make it all the way to the actual ending, rush the ending, and don't rush it because it's axed, but because of bad writing.
Tbh, if the OG Author didn't bother fixing the ending when the WN got published as a LN, do you really think it would actually bother with fixing it years later after this was adapted to manga until it ended?, it didn't bother with previous adaptations, it wasn't going to bother with this one, probably won't bother with the next one to end either, whoever the endings are, it was the ending for the Author and it didn't look back, specially not when it finished like 3-4 years before it got adapted, once it become a LN and then adapted, looking stuff like this is more on the Editors to not let this kind of thing pass and giving the ok, this is just purely bad writing and not fully committing to your work
Beside, how often are OG Authors actually involved in manga adaptations, probably not as often or as much as you think they are, specially not this one when is writing that many other works at once to bother with an adaptation of an old finished work between many of their other works that are being adapted, adaptations purely coming from WN, probably authors are more involved and can change more stuff since a WN is still more of a "draft"
This is a thing too, with many adaptations and people calling "axe" on them, but they tend to forget that many of those ones, start and are used purely because of promo motives for their respective LN, bound to end early from the start after adapting their given milestone, or there is the rare case where the manga end up being more important than the source, obviously there is those that actually do get the axe for various motives