/u/RiotKatana I rarely comment on these criticism threads and I am not one to shy away from spending money on League content, but I just want to voice a few specific points that are stopping me personally from buying this skin:
TL;DR:
1. Forcing the user to buy the bundle instead of a more flexibile pricing model
2. Marketing the skin as Ultimate, as opposed to just Quest
3. The base model does not look very good, so having the only other option being the Ultimate skin feels terrible from a player perspective
4. People already don't like her kit/the announcement, so some of the hate is probably spilling over
Forcing the user to buy the entire bundle, as opposed to a pricing model similar to Star Guardian/Pajama Guardian or Cosmic/Dark Star Lux. I can justify spending $10-15 for one of these skins based on the splash arts I have seen so far, but $25 for 2 others that I probably won't use (as you said, people generally just stay with the form they like anyway)? That has to be a very exceptional skin, and right now I am not seeing how that translates in this concept where I can only change the skin in champ select. Especially after the release of some great work on skins like Spirit Blossom Thresh, Spirit Blossom Ahri, and Storm Dragon Lee Sin.
Marketing it as an ultimate skin sets the user up for disappointment, especially since the bar is now Elementalist Lux for that tier of skin. While I completely respect why you cannot make that level of skin from a technical/game health perspective, it also should be recognized that Riot has set the bar high for Ultimate skins. I think if this was just branded as a Quest skin, instead of an Ultimate skin AND Quest skin, the backlash would be harsh still (grinding to unlock a skin I already paid for doesn't sit well with me personally), but less severe.
I, like a lot of people here, dislike the base model of Sera (for me the colors are too bright on her outfit, wish it was more subtle/Piltover-style with her hair being the big color pop, more similar to her final form in KDA). As someone who plays supportive mids, I still want to play the champ and would be willing to pay $10 for the release skin if it corrected some of the color scheme problems I had (Especially if the color just changes on her abilities from the light blue-->pink in the skin line), but $25 is just too much for a champ I have never even played(see point 1).
People are already mad at the kit of the champion for her "similarities" to Sona and the amount of hype/effort that went into marketing Sera for what is admittedly an underwhelming release announcement and pretty generic character (although I am in the camp that she will play much differently than Sona). I am pretty sure that if people were super hyped to play Sera and you announced this plan, the feedback would be less severe.
This is obviously just my opinion from what I have seen so far and I am totally willing to admit fault if the skins blow us away on the PBE. Just thought I would offer some constructive criticism since a lot of these threads can devolve into "RIOT BAD" without articulating why. I think this is more of a marketing/branding failure than anything else and I appreciate you engaging with the community on this!