Complex sentence: Seattle has set MLS records for average attendance, led the league in season ticket sales, qualified for the MLS Cup Playoffs in each of its first seven seasons, and reached the MLS Cup Final for the first time following the 2016 season.
Split into simpler sentences: Seattle has set MLS records for average attendance, led the league in season ticket sales, and qualified for the MLS Cup Playoffs in each of its first seven seasons. Seattle reached the MLS Cup Final for the first time following the 2016 season.