I know that in my country and I.1 it is around 1 of 4 new teams staying above. I think it should be around 50% maybe slightly over. And that is because the relgation penalties. That thing shouldn't be in the game at all I think. Many clubs relegate right after they've been promoted in the first year. Although they tend to/have to have similar rosters to slighlty below average (it depends on a country I guess) Div 1 team they are punished mightily if they fail to stay above, so it is hardly beneficial to be promoted then. They were being on their way to be competititve and now that choice is a bit of given away because of that harsh penalties. I don't think 20% is that high cause if they don't deserve to stqy above, they would be probably flushed away to Div 2 by the next wave of rookies. Without harsh penalties for relegating team bigger roatiton wouldn't be a problem.
I know from my former Div 2 that there were 2 new teams (or re-promoted appearing in the same league again) from 4 (or 5 in one year) staying above for the period of my existence there. I didn't see much of a problem, weaker roookie terams ended up 7th or 8th and stromnger ones starte to challenge old teams of the league. That's in a country existing from Season 3 with 4 levels of competitiion, so some newly Div2 are quite new. But sooner or later we will reach equilibrum worldwide and then such al eague would be more challenging if those ration of new teams staying above would be kept - which probably won't be the case as the game tends to "closingů the leagues from top to the bottom slowly but steadily. If there wasn't artificial gaps among levels, with equilibrum reached league with 2/4 rookies staying above would be more as there won't be as much treadmilling teams as there is now.
Other problem is unbalanced leagues. In my country there is 25% of bad luck ending in big 8 where 7 from TOP 9 teams are and if you are in Div 1 first time you have no chance to stay above there - which leads to 3 (6th to 8th) relegating teams of Great 8 insterad of 2 and 2 making it even harder for 2 rookies to stay above as they are competing for 5 th place to be saved. if you keep those relegating penalties, with current salaries, probably even experuinced fromer Div 1 team promoting to that strong 7 in Big 8 would be face against mission impossible because as you wish he couldn't kep his player in Div2 thus losing competiveness against top dogs.
For example I expect my gate receipts to fall from just over 500k to below 250k. That is not realistic and i don't see a purpose for that. I admit I got promoted underdeveloped not having top roster yet but you want to promote if you got a chance (and I got to the strong 7 anyway, not complaining, it happens).