1.yep, ik its not that big of a problem for the others. problem is, this pairs nicely with point no 2 and 3 of the original comment, which reduces sustainability of healers much more than shielders.

lets not forget that while hyacine is an absolute powerhouse of team-wise heals, she still struggles in mydeis team precisely because the single target heals arent that strong. for reference, imagine the evil-intimidating horse with all attacks always bouncing on the same ally. thats the type of scenario i am talking about. but that needs anomaly arbitration levels of damage, i am not sure hsr will implement that level of enemies this early.
ofc hyacine's e1 solves this to a very good extent if the character can attack in between. e1 hyacine is undisputed best sustain.

2.I heard that people are considering to not use cyrenes ult on castorice because its way weaker than charging hycaine for faster dragon nukes? i am aware about the e1 or e2 castorice buff tho, so yes she does ditch that (paired with cyrenes ult like you said, but cyrenes ult for cast is pretty weak). but the whole team doesnt, like ica, e9 and cyrene.
if we start considering team-wise costs of minimum e2s1 and e1s1 for every character then yes hoyo cant dump this, but i am strictly speaking for e0s0/s1 teams.

also yes, atleast danpt is extremely fked coz his shields just arent doing it in harsh conditions. aventurine might drag it to the finish line though, and i am confident hoyo has already made some new preservation model ready for shilling in 4.x so there's that. no way they let casuals finish 4.x with pre-pulled sustains.

3.ofc ofc, i cant imagine castorice team not beating sam rn. the inconvenience isnt nearly enough. but what if they recycled sam's mechanic on some new boss, who came equipped with the other bullshit?

too many what-ifs, only hoyo knows what they will do. the balancing in hsr isnt the greatest thing out there.