According to Toyota Motor North America CEO Jim Lentz:
https://www.autonews.com/sales/toyota-letting-sienna-sales-fade-until-redesign
Also, Jan 2019 sales numbers for Sienna (-41.8%) were terrible:
https://www.wardsauto.com/industry/toyota-and-nissan-fall-honda-inches-january-us-sales
Edit: Overall in 2018, Sienna sales were down 21% - lowest since 2009
https://www.autonews.com/sales/toyota-letting-sienna-sales-fade-until-redesign
Also, Jan 2019 sales numbers for Sienna (-41.8%) were terrible:
https://www.wardsauto.com/industry/toyota-and-nissan-fall-honda-inches-january-us-sales
Edit: Overall in 2018, Sienna sales were down 21% - lowest since 2009