Is it really a good idea to base the assumptions of a game on 20% of the audience?
It may be when those are the tables generating the most new players. I have two ongoing campaigns: On has run for 60 sessions over 3 years and included 7 players of whom 5 are still playing.
The other has run for 20 sessions over a year and a half and has included 24 different players. Of those 24 players, 6 of them were entirely new to roleplaying games and 8 hadn't played in 3+ years. Some of these players have stuck around and continued participating in the campaign. Others drop in once every few months as their schedule allows. Others haven't stayed interested. Others have spun off and begun playing and running their own games.
What's the difference? Well, the former campaign has a tight, ongoing continuity. Joining that campaign is a major commitment: You have to be willing to compromise your personal schedule in order to coordinate game sessions with a half dozen other busy people. We need to have a fair degree of confidence that (a) you'll be a good fit for the campaign and (b) that you won't drop out and cause continuity problems when your character vanishes. So inviting new people to play the game is difficult, and inviting people completely new to roleplaying games is essentially impossible.
OTOH, the other campaign is a looser affair: Whoever shows up for a particular session forms the adventuring party for that adventure. When the session is done, all the PCs head back to town ready for the next hodgepodge adventuring party that goes out to the dungeon or looks to explore the nearby wilderness. It's an old school structure, and it means getting new people involved in the game is as easy as saying, "Hey, you wanna play?"
If all you care about are the die-hards who play the game on a regular, weekly or bi-weekly basis then the latter campaign structure isn't where you want to be focusing your attention. The "retention rate", as you say, is terrible.
But if football manufacturers decided that the "retention rate" of people playing catch in the backyard was terrible because most of those people didn't join peewee leagues... Well, they'd sell a lot fewer footballs.