Usually game publishers don’t release new video games for a franchise due to low popularity or sales. We’ve seen this in the past as EA is not making another Mass Effect video game thanks to the ill reception Mass Effect Andromeda received last year. Likewise we won’t be seeing any new Rock Band or Guitar Hero games either due to low sales.
That being said, there are rare examples that sequels or new games are not released because the original title is super popular. There’s a reason we have not gotten Minecraft 2 or Grand Theft Auto VI because Minecraft and Grand Theft Auto V are still selling like hotcakes.
It seems as if Ubisoft is putting Rainbow Six Siege in the same category because Siege has become a surprise hit. The game launched a few years ago, and was criticized at release due to a lack of content and no single player campaign. Although the game still has no single player campaign, Ubisoft was smart enough to keep adding new content and the game has more players now than ever before.
Ubisoft announced recently at the Rainbow Six 2018 Invitational that there are no current plans to release a sequel. This is because Siege has amassed 27 million players as of December 2017 which is a huge number! To make the existing fans happy, Ubisoft plans to support the game for ten years (up to 2025) by eventually adding over 100 playable characters/operators.
The online community is what saved Rainbow Six Siege from falling into oblivion. Its realistic style of gameplay is what separates itself from more fast paced shooters out there like Call of Duty or Battlefield just to name a few.