find it amusing how so many seem to think a "DLC/Content patch" such as characters music etc, and "balance" patch, are the same....
sakurai made it pretty clear he's not adding any more DLC such as characters, new music, stages, etc.
However not once did he say they wouldn't continue to adjust character balance later on. They kind of have to keep an eye on the game and at least make some tweaks to characters here and there after they drop two new characters in. They likely waited to see how much corrin and bayo effected things, whether from reading competitive players input at places like here, or surveys in japan, seeing actual tournament results from japan/here, or hell just even looking at online matches, possibly all of the above.
Sakurai has to get some sort of data together to see what he thinks might need to be changed. Whether or not he does the right type of changes to characters is always up to debate, but regardless, he doesn't just randomly decide one day that "hey, i think i should change this character now" with no research.