If Baltasar Carlos is anything like his double-first cousin - and it stands to reason that he has a 50% chance he could be - Europe's gonna look a whole lot different.
For example, in Philip IV's waning years Baltasar Carlos is gonna be a young man - probably if anything like Louis XIV - with a desire to prove himself, and may well be the sort of warrior-king Spain hasn't seen in a while.
Point being, if he's inherited any of Philip II and Henri IV's genes he's gonna be a warrior-statesman, who just might stop Spain's decline (or at least slow it down).
In the thread All Hail Germania an Emperor Baltasar - who has more or less the same birth-year and parentage - becomes one of the greatest kings in Europe. Although, the thread admits to being a Germanwank and a Hapsburgwank.