The tournaments are the only way to earn the tokens for upgrading the banners from the season tournaments.
^ This. These banners are incredibly powerful, and thus are only upgradable through the tournaments (and success in them)
Also, the Banners read 2019 War Season; are there plans to release a new furnishing and new banner set with 2020? There are two more, maybe three more wars in 2019.. will the Sovereign tokens remain the same?