Given the fact that they've rescheduled all of 2022's cruises to St. Petersburg, one would certainly hope that all passengers are aware of the possibility that St. Petersburg will need to be dropped next year also. However, there will probably be a few very confused people who will be shocked and angry at the cruise line when they find out. These are the same people who show up with their kids at embarkation and didn't realize they were supposed to register - and pay for - their children ahead of time. But there should be much less turmoil and fewer disappointed passengers in 2023 than there were for the 2022 season, because everyone has a year to decide if they would like to take the Baltic cruise without St. Petersburg or an alternative cruise.
St. Petersburg absolutely was a highlight for me, but the alternative ports might be really interesting. Not sure it would be worth it to travel all the way from the US without St. Petersburg, but I think it depends upon whether you think St. Petersburg will reopen within the next few years, and how likely you are to be able to repeat this cruise (Stockholm, Tallinn and Copenhagen would absolutely be worth repeating if you have the money and time).
If no one answers your question here with Princess specific info, the various alternatives utilized by all cruise lines have been discussed on the Baltic forum.