blowing up the bridges will cut the ukrainians east of the river off from their supply lines and allow the russians to relatively rapidly reach the dnieper, which is where they should stop, at least for now.
it will be up to nato to prove that they are not a continuing threat to russia. if they can do that, an uneasy peace is possible. if nato continues to pose a threat to russia, russia will remain justified in moving to the next river and the next mountain chain.
the natural defence of the carpathians, which is the source of the vistula, are the maximal boundary points for this russian defensive advance but it never made sense for them to think they could do it all at once. it was always crazy.
stopping at the dnieper for now will give them time to rebuild and give them time to talk it out.
ukraine is not a real country. the russians are on relatively strong historical and cultural grounds in annexing any region to the east of the dnieper, but the historical boundaries of austria and poland start to pop up when you get into the area between the dnieper and the mountains and the issues become more live and more real.
the government in kiev made this choice. it walked down this path. it will need to sleep in the bed it made for itself.
a divided kiev may be real life for the next several decades until the next generation decides how to sort this out.