Let's compare these two directly from the details you've given. Correct me if I'm wrong here, but:
Bubbler Redux:
- 15f activation (.25s)
- 6s duration
- 100HP (+extra from big hits)
- object damage multipliers apply
- knockback on hit, varies by shot intensity
- (presumably) a larger hitbox
- chains when touched
Ink Armor:
- 30f (.5s) movement lock, 1.5s activation (2s total activation time). SPU can bring total time down to 1s but cannot lower movement lock
- 6s duration (9s max SPU)
- 30HP
- knockback + damage above 100HP pierces up to 80HP
- universally given to all teammates
I don't see how this isn't Ink Armor 2 with more extremes. It doesn't instantly go to your entire team - neat. Even if it basically always means backline won't have it, it's not exactly tough for front and midlines to get close enough to give each other the bubble for a push (assuming that mechanic works like it did in Splatoon 1). Now every weapon in the game needs to deal enough damage to splat twice instead of dealing only 30 extra - but because of object damage multipliers, some will have to deal even more than that? And that's not even mentioning that with that activation time, this is a panic button. Even Trizooka requires more startup time! I can't find frame time for Bubbler activation in 1 and looking back at footage suggests it's instant. 1⁄4 of a second is better than that, sure, but the original version only lasted 4.5 seconds without SDU - something that starts quicker with much more health than Ink Armor that lasts just as long sounds like a huge nuisance, even if it isn't straight-up invincibility.
I simply don't think the new upsides are nearly enough to justify an even more annoying version of Ink Armor with the slight benefits of being able to knock it around and a larger hitbox. You say it's balanced by the fact that every weapon is required to double their TTK to counter it, but that just makes weapons with slow TTKs worse instead of almost universally rendering three classes worse. I guess it's an improvement, but to what end? If you're counting on object damage multipliers to even the playing field, you'll end up either with the slower rapid-fire weapons at a disadvantage (at that universal 0.8x suggested) or functionally giving this thing so much more health to compensate for TTK differences that every weapon hates it equally. Ultimately, I don't see this being fun to engage with at a high level - never mind how infuriating this would be in a casual setting.
If nothing else, doing the legwork for this thread has really given me a deeper appreciation for the work the devs have put in trying to balance the concept game-over-game because it is absolutely hellishly difficult to get the concept right at all, let alone balance it for the competitive and casual side of the playerbase. It's not like Nintendo always gets it right, but going through the update history for all of the shielding specials does paint a clear picture of how we got to where things are today and why they ended up landing on Big Bubbler.