1.1 Steamspy is no longer and hasn't been a realiable sorce of infomation. The creator itself has told everyone Steamspy is not always correct but people think it is the correct infomation. As of now we do not know how many games are being approved and removed. We do know games are getting remove and added but how much of a number is not yet answered. You could count manually on their seach results but you have to keep up with ever changing results. Steamspy may not have adult results in their database because steam hide them or is not ok to show in public. In simple terms you can not relay on Steamspy.
1.2 Nope, people will buy the game if they like it (every COD, AC, TCG, Pokemon, FIFA, etc). Even some of developers here who sell more than $10 have found people buying their games or art: https://epicstockmedia.itch.io/puzzle-game
1.4 Itch.io allows developers to upload their apps in what would have cost people $100/yr in Apple or $25 google play fee to upload games. Also itch.io doesn't need to follow google play or apple app guidelines which can open up to developers who would otherwise couldent upload to google play/apple. There is no threat of itch.io losing to apple or google play. Console is console not PC, i haven't seen anyone upload a console version in itch.io but such a thing is impossable considering todays console of Xbox and PS4 require the game to be in their store, this doesn't change much for anyone.
1.5 itch.io is always getting better, it's main advantage over steam is better communtaion with itch.io, how much percentage you willing to give, lots of games are DRM-free, Game pages are customzie, uploading more than just games, etc. No one offers better things or gets better stuff than itch.io
2.1.1 Let me tell you some things about steam, no it is not the largest, no numbers have reveal such things not even from Valve or Gelb Nuel. The myth started with developers, who in the past saw steam has a gaming platfourm for relasing good games. After greenlight it become a mess thus starting the myth of largest gaming platfourm for allowing games instead of real numbers backing it up. Trust is also not true either, many developers even before greenlight did not trust it. When it become populor the people liked the game not the company who publish it, therefore the customer trusted the developer not Valve. Records and news shows Valve is not trusted by customers, everyone always has a reason to never trust Valve. Also no it is no 30% cut, no one knows the real number because Valve makes developers sign A non-disclosure agreement (NDA). Everyone at one point did ask what developers are getting but no one answered, one person who no one can find said 30% then everyone belived him. That is the whole story of how Valve did nothing and somehow became one of the most deceptive of any company known to man.
3.2.1 Again people will spent money on games and projects they like, sometimes a little bit of marketing pushes people but not all the time. The myth of itch.io being developer focused platform is from the news of new features being added for developers instead of gamers getting new stuff for them like reginal pricing, cloud saving, mod support, etc.
3.2.2 Steam also has a problem, they allow any game to be in their store. I am sure you already heard of this from many news sites and post about it.
3.2.3 itch.io is known by developers who use itch.io for pre-launch to steam and gamers who could not find games in steam they liked or gamers who can not find free games. Tabletop is being very populor in itch.io right now and it could become THE place to buy tabletop games made by other people.
3.3 & 3.4 nope because again itch.io is doing something while other companys are not. Each of the listed company you talked about is in a real threat of the goverment while itch.io and developers might not. Gamejolt has a problem with fanmade games, Kartridge is only Windows and Mac, many linux users go to itch.io or steam for games. GOG has a problem with itself.
I think you need to do more reserch, this essay screams 'someone who did not reserch enough.' Your infomation is too related to developers who understand one side not the whole story or infomation that comes with it.