It's normal for MSNBC's ratings to go down for the first few months after an election. MSNBC's ratings always go down when political news dies down or is depressing for Democrats. In this situation it's both. Political news is less interesting because the election is over, nothing is happening. Yes Trump is making some extreme cabinet picks but they aren't going to actually do anything over the next 2 months. So the political news is not just boring and unimportant, it's also depressing for Democrats because Trump won. It's really not surprising Democrats have no interest in watching cable news for the next few months. Ratings also crashed this summer when someone tried to shoot Trump and they switched to a pro-Trump NBC News feed for a day. Ratings even fall when there is big non-political news happening like a hurricane. It was different when Biden won because Trump didn't concede which was an unprecedented major political event.
When Trump takes power and actually starts doing things, ratings will go back up. During the first Trump administration, viewers turned to MSNBC for all the chaos Trump caused. Remember when he separated families and children from their parents at the border? They had almost wall to wall coverage, multiple MSNBC shows including the primetime shows where anchored live on location outside Trump's child prison camps near the border. They spent hours and hours covering all the indictments and criminal trials of Trump campaign officials. They extensively covered his ties to Russia. From almost starting a war with the dictator of Iran to visiting the dictator of North Korea, Trump gave MSNBC endless hours of special coverage. After Biden won, the times when MSNBC had the most viewers was covering the troubles of Republicans & Donald Trump. The January 6th investigations, the 2nd Trump impeachment, the Trump indictments, the Trump trials, etc.. etc..
I'm sure they are already working on special graphics and theme music for all the things that are going to happen. Democrats might not be tuning in now, but they will be tuning in when MSNBC has wall to wall coverage of mass deportations and all the other chaos that a 2nd Trump term will bring.
As for advertising, subscription fees are a huge part of the revenue for cable news networks.