While I do concede that the points you've brought up themselves are overdone, I don't dislike it. Not always at least.
People nowadays have the attention spam of a coked up hummingbird, so slow and gradual means not getting support - unless you're doing it for the love of the game. Pun intended.
That's the main reason you have non-incestuous members of a harem. It's easy and easily explainable access to early sex scenes.
Now the points
1. Wouldn't make sense to have a bunch of gorgeous women fucking and having children with Quasimodo's retarded brother, does it? But sure, they could be average looking, and make up for it with their personality etc.
The issue comes with renders or drawings. Even in romance Anime this issue arises, namely: you can usually only make either a good looking, or a horrible looking MC. The average kind of isn't seen. Even if a guy in a game or an Anime says "I'm just your average looking guy", you won't see any difference between him, and the rest of the male cast, unless they are made to be either Adonises or Ugly Bastards.
1.5. Dick size I agree is usually weird, unless it's made a running gag (Grandma's House) or there is a "scientific" explanation or cause. Same thing with those disgusting bimbo-pornstar tits on everyone.
2. It's either to remove a Netori element, or to remove guilt. Either you will have an asshole mom/wife and MC that cucks a genuinely good man and father, or you will have an abusive asshole of a man and father that deserves to get cucked.
2.5. Ofc they fuck him, it's an AVN, a PORN game. Why make a game with incest, where the family members DON'T fuck? That makes even less sense.
3. Yeah, I would like to see some disparities here, that setup is too predictable, at least nowadays. Sometimes the big sister is just a Tsundere, and not a complete bitch, but it makes little difference.
I've also seen it flipped, making the little sister a spoiled princess kind of deal, and then you instead have the older sister be a sensible and loving Onee.
4. Harkens back a bit to the first part I wrote. It's about support or having SOMETHING happen. You either have a slow burn, that tends to go nowhere, with some teasing, sudden sex, and then the game ends. Or you have a grindy sandbox game where you have to to X things X times in order to get a new 'stage', or you get teasing/handjob, and do that 15 times (only once per day, though!) to get to blowjob, etc.
5. Again, point 2.5 (porn game). Adding here that devs will get criticized or abandoned if they put lovely ladies in the game (esp tagged as Harem), and you can't fuck any of them. Harem sells.
6. Yes, I do agree here. I'm a bit tired of the perfect bodies (with pornstar tits). I would like to see a chubby younger sister learning to love, or a mom that let herself go a little after her husband cheated on her and left her. Now ofc lolis gonna loli, but it's rare to see a chubby loli.
I don't mean a fucking meat mountain here, but a little chub (without cow tits, ffs) would be refreshing.
