Determinism isn't always a bad thing - there are some alternate worlds like say the Germans winning the Second World War that are also very unlikely (because, yes), but also very negative for the vast proportion of humanity. Sticking by a "because it failed" isn't as negative a statement for them as it would be with other societies.
To me it depends on how many layers of change you need to impose on a system in-order to make something survive, thrive or fail. Poland-Lithuania is fascinating, but part of the reason why it is fascinating is because the state is constitutionally a bit bonkers and might not be as interesting to people if it was 'fixed' to survive as it is because of the quirks that ensured its failure. If we are going to apply the layers of change theory to Poland-Lithuania, then how many degrees of change do we need to apply for them to make it? Quite a few. I agree,
@Flavius Iulius Maiorianus, Sweden succeeding in the Great Northern War would be good for Poland, but then if we are rethinking things, hasn't the problems that doomed the PLC already manifested and doesn't that need to be addressed? Do we even need to have a union of Poland and Lithuania given that these are two different but related cultures? We talk a fair amount about Scotland and Ireland avoiding the UK (I don't know how to phrase that in a more politically neutral way), perhaps there is no union and therefore Poland is able to focus on itself, further away from Russia and survives and Lithuania is closer to Russia and doesn't?
Whatever the case, I do think this has to be earlier than the terrible trio and contemporaries Peter the Great, Frederick II and Maria Theresa to give them the best possible chance, I think the POD has to be 16th or first half of the 17th century, when a lot of other countries are going through state-building institutional journeys to give them time so that when these hugely important figures appear on the scene, they are actually in a good place to sustain themselves going forward.