Big Brother is a summer show, but CBS has produced a winter/spring season in the past due to a lack of scripted content... and could do it again in 2021. It is certainly true that the show has been in a steady decline in recent years when it comes to ratings. For reference, seasons between BB1 and BB5 averaged over 8 million viewers per episode. Between BB6 and BB13, the show brought in an average of 7 million viewers per episode. Then, starting on BB14 and all the way up to BB19, the show reached over 6 million viewers on average. The network saw a pretty significant decline in ratings for BB21, which had around 4 million viewers per episode.
For four weeks straight, the Big Brother 22: All-Stars season consistently struggled to reach 4 million viewers, but it finally found its footing around week 5. As of this writing, week 8 - which included the Triple Eviction - has had the best BB22 ratings. Still, the return of reality shows and live sports are running CBS at the moment. It's much harder to quarantine so many actors for so many scripted shows, so most of those productions have been paused. Meanwhile, new seasons of Big Brother and Love Island USA were made possible despite the pandemic.
Historically, Big Brother 9 is the only regular new season to air between the winter/spring months, but the circumstances for that to happen were very similar to what the networks face in 2020. It comes down to a generalized lack of new content for networks to broadcast to its viewers. Between November 2007 and February 2008, the Writers Guild of America had a notorious strike. In other words, every single writer that worked on a fictional series on TV went on strike for months. At the time, several shows didn't come back from their mid-season breaks. This prompted the networks to invest heavily in reality shows, as most of them can do without an actual team of writers. And it was exactly that circumstance that prompted CBS to produce three Big Brother seasons in the span of 14 months: BB8 (July 2007 - September 2007), BB9 (February 2008 - April 2008), and BB10 (July 2008 - September 2008).
When analyzing the precedent that CBS set for Big Brother, there is certainly the possibility that Big Brother 23 could come as early as winter/spring of 2021. The network is one again scrambling for new content to broadcast, but most scripted shows are either limiting their production or not returning at all for the time being. This means that CBS only has news, live sports, and reality shows to fill their schedule. It's easy to see how the network is already being strategic about this: they've held onto The Amazing Race 32 for months in order to fill the void that Big Brother 22 is leaving once it ends this October.
The jury is still out on whether the fans will get a new Big Brother season much earlier in 2021 than they expected. The lower ratings for BB22 definitely contradict this possibility, but the lack of scripted content could mean that CBS might still be better off producing a new season of the show than having an empty schedule.
Big Brother 22: All-Stars airs Mondays, Wednesdays, and Thursdays at 8pm EST on CBS.