Flat math is here to stay. When you look at how designs are trending, games designed after 5e have even flatter math. Draw Steel, Fabula Ultima, The Stormlight Archive Roleplaying Game, the explosion of Powered by the Apocalypse and Forged in the Dark games, Year Zero Engine game, newer 2d20 games, etc. All have flatter math than 5e, some much flatter. Also, in the case Legend of the Five Rings, Warhammer - The Old World and most iterations of existing games they tend to have much flatter math than their predecessors.
Flatter math is easier to design around, easier to balance and often easier to teach to players. It's also just more resilient to a variety of situations.
The only game that really breaks the trend that comes to mind is Pathfinder Second Edition, but that is a little misleading in that it's fairly flat within the scope of play since almost all the gains in accuracy are level based and you are expected to keep play within a -4 to +4 level band of the characters.