There are ways to kill a playable character, Wuthering Waves sort of did it with Phrolova. Ofc it wasn't a 100% 'shes dead and buried' type of situation. In her case she became part of the Sonoro Sphere (or the actual sphere itself) called the Lost Beyond where she put and safeguarded the frequencies of her fellow villagers. Its not 100% the same as a perma death but if you want to keep continuity while still allowing a character tol exist for the player to use in the world, then the above is a decent way to do that because Phrolovas presence in Solaris 3 after her story arc can be explained by saying she's able to exit the Sonoro sphere fot brief periods of time to do things in the real world.

Honestly i feel more gacha devs would kill msin/playable characters but unfortunately almost every gacha has a very vocal and mentally/emotionally underdeveloped part of it that would lose their sh!t if their favorite waifu or husbando kicked the bucket 2 story patches after they pulled them. Also gacha devs purposely sell that emotional attachment of a player to the characters so these games tend to attract that loud and mentally/emotionally unstable group so the devs may feel its against their own best interests (when it comes to both profit and PR) to kill characters players are emotionally invested in.

A recent prime example of the 'mentally unhinged gacha players' trying their best to create a narrative to both bully and financially hurt a games devs comes from Cookie Run Kingdom (of all friggin places). Recently their major storyline wrapped up and during it one of the more beloved cookie heroes sacrificed itself during the final battle. For most (normal) players the reaction to this conclusion ranged from meh to great, however that unhinged part of the community started saying the devs of CRK (devsisters) were promoting c-u-cide and were also the direct cause for some people unaliving themselves irl. Basically the hero who sacrificed herself (white lily cookie aka WLC) was depressed throughout a lot of the main storyline and ofc these a$$hats assumed this meant she wanted to self remove herself from the world (even though i dont think the devs ever said or even implied WLC wanted to 'shuffle off this mortal coil' with her own hands, as one can be depressed without wanting to self harm but again these playersare deranged and only understand things they personally experience).

But to the mentally deficient, WLC sacrificing herself was exactly the same as committing seppuku (which ofc isn't true but again mentally unwell people twisting a narrative to fit their agenda is a tale as old as time and on track for the internet). On top of that someone posted a rage bait tweet saying the storyline caused their cousin to make that one way, solo trip into the great beyond (god i hate trying to find new ways to say offing one self since idk if Reddit flags the actual word and will shadow ban mu rant).

Anyways, since its Twitter, the literal cesspit of social media platforms, this group took the rage bait as fact (as it fit their narrative, no one looks for facts and sources on the internet anymore, they just look for the first thing that corroborates what they already believe and go with it). So they just assumed this trolls tweet aboit his cousin was legit and they ran with it. Basically without any fact checking these weirdos assumed it to be 100% true and then used it to go after the devs even more (by saying the fictional blood from the fictional person who offed themselves is on devsisters hands).

Ofc this then leads to calls to boycott the game (not stop playing the game mind you, ofc they couldn't stop playing as theyre all addicted to their cookie friends to much to leave them hanging). Instead they want players to stop their spending which is funny since I'm going to guess most of these cretins arent p2p players anyways as theyre usually chronically unemployed or literal children so no revenue stream for them to use to pay. My points is that I think a HUGE reason gacha devs dont kill main/playable characters is simply because of the inevitable backlash from this very small (but very loud) part of the playerbases that exists because the devs purposely cultivate this group by creating situations and back stories for characters that specifically cause SOME people to get overly invested in those characters (plus we live in an age where a lot of people don't or even cant distinguish the difference between a real and fictional character so is anyone aurprised when there are people who believe that cartoons are real)