I thought I would answer the questions you asked to the best I could, granted I have only bin playing league for a year now with a few games sprinkled in 2016 but never really played then till last year so if I am wrong about something I am sorry and I did leave out some questions because I just don't or can't answer the question.
Why isn't player feedback taken into consideration?
A: I am sure it is but when you have 1000s of people asking you questions or making comments its hard to get back to everyone. So even tho it looks like they are not responding I am sure they are.
Why only the fun things are noticed and removed, while the unfun things are amplified?
A: I wonder what they removed if your talking about blitz or AURF riot had gone on record many times in videos saying that these are special event maps and also their stats have showed that when these come out even tho people love them they notice a drop in players because and I quote "even tho the game is fun it seams to make people also want to stop playing the game as well".
Why was Nexus Blitz removed?
A: Same as what I said before in the question above, and also to add riots main games are Summons rift Aram and Twitsted Treeline and that's how they seam to like it.
Why am I put on a 10+ game losing streak after 4 game winstreak?
A: This to my understand has to do with your mmr if your mmr is lower then the rank you are in it will punish you because their system sees how you play (your fault or not) in your games and thinks you don't belong in your rank so it works against you to get there. Annoying yes but this is the system they like that seams to work for their needs.
Why do I get above mentioned trolls in my promo games?
A: I get them to all the time in bronze all the way to silver where I am now but my husband has climbed from silver and he is in plat now it never stops trust me. No matter what rank you are in you will always find trolls, feeders, toxic people, Salty people, Afk'ers, I need to farm people. Also if it makes you feel better at least you did not have a game like me. And I reported the trolls and as far as I looked they are still playing to this day even tho they did this in my game. I never saw any punishment (unless it was a mute or a leaver buff) for any of them and I dont feel this is fair im not saying I played any better then them but Jax is not an adc all I have to say.
Game with the trolls/feeders -> https://ibb.co/Wz1JMNf
Why do I earn 15 LP per win and lose 21 LP per loss in Gold?
A: It is your mmr that chooses this, it is based on how well you play not just how many wins and losses you have.
Why does the damage meta even exist?
It is the sole reason why people are quitting the game.
A: I would not say it is the sole reason on why players are leaving because the meta changes as the game does and the items in the game to be more exact. Before the did the buff to infinity edge bot lane was a gong show and this was back when worlds was going on. And also look at now people in pro play are playing Sona Taric bot lane and it works it seams. The Meta is just something you work with I mean I played urgot pre nurf then buff and then nurf that just happened like 2 weeks ago.
Why are runepages so expensive?
A: This is a free to play game they give you 2 rune pages and that is all you really need just change your runes as your picking your champ. If you want more choke up the crap ton of be or get some rito points.
Why are champions so expensive?
A: Its a free to play game this is where Riot makes their money you want the game to stay free to play then this is how it works.
Why every second patch has to include a skin?
A: Its not every 2nd patch that mean they are making a new skin every month most times they launch skins to go with an event they do, in 6 months they have had 10 skin launches with different skins in the launches. Yes that is a lot of skins but this was around when worlds and since then we have had Christmas, New years, Valentines day the Chinese new year. Every skin they have launched has come with an event that gives players more quests free items in the form or boxes and orbs and free event currency to use to get most of the items they have in the store for the event minus the skins themselves (Sometimes). I would think that having more skins would be a good thing not a bad thing its not like you have to get them its just riots way to make money on a game that is Free to Play.
Why don't we have the option to choose between legacy and new client?
A: I think riot has said this in a video but the reason why you cant was because the old client was outdated and clunky and it was too difficult to update an outdated client when they could make a new and better one. Its not that big of a deal the new client has way more options and it better and runs faster then the old one anyways.
Instead we have this "Low spec mode" option that barely helps people with weaker PCs
A: Just be happy Riot has a "Low spec mode" other games like Paladins, Overwatch, World of Warcraft, Guild Wars and so on don't even have that option you run it on low or don't run it at all. And I have played league on a laptop that I got almost 10 years ago I upgraded that thing from windows 8 to 8.5 that is how old this beast is and it still runs I have to have low graphics but it runs. Unlike all those other games I listed I cant play but league I can.