you would do better to just give it to everyone as a third summoner's, than try to nerf it slowly out of existence
1. flash makes the game more interesting, not less so. People complain about it because its confusing but without it engagements get dangerously linear and boring
2. it radically changes the balance of the champions. Built-in flashes get much stronger every time flash gets nerfed. its strange how slowly and *prudently* you move in balancing individual champs and items, and how quickly and *imprudently* you move in changing summoner spells. Most people may not get it so complain less but it still makes the player experience much more bumpy, and if you want people to feel they are investing in learning a skill, that's bad.