It works in the US because you have closed leagues. But with the relegation system, you can still have something to play for even if you're 14th out of 20.

The biggest downside with the play-off system is that if you have a shitty season, you have literally nothing to play for. Whereas in Europe, a shitty season means struggling until the end to be able to stay in the division. It can result in late-season shockers and the joy of staying up is sometimes akin to winning an actual title.

Furthermore, it will greatly diminish the regular season games. And the big teams will still prevail in the end, nothing much would change (as proven by National Cup winners over the years who are rarely smaller teams). The reason it works in the US for diversification of winners is because you have laws in order to level out the playing field (salary cap, drafting system with the bottom picking the first players...), not because you have play off.