The solution is to pay for 1st class. That is what I do. And you can get 1st class supersaver tickets up to 1 hour before departure.
Regarding capacity: There are actually trains where the 1st class is very well filled. SBB uses standard consists for its trains, and these are sized to average expectations.
Given how good value the SBB really is I think that you should not complain to much for the occasional full train.