Only that you just taught the already aggressive ruling elite of a huge country with an abundance of resources who don't care about anyone including their own except that they need them for work and for the fighting that threatening use of nukes gets them anything they want. Moldova next - it's not EU or NATO, already very low risk for Russia, if they can get there. Which was (is) a stated goal for the current war, to get the entire south of Ukraine to take away their sea ports and to get to Moldova.
They'll try the Baltic states next. Not a full invasion, just lots of little aggressive actions. Even previously they did murders in the EU, financing of radical parties out to undermine current EU country governments, supported by propaganda. I don't know how much it actually influenced US elections, but I think it's save to say they at least tried.
Giving them Ukraine will be massive. They will also have lots more of the oil and gas reserves under Ukraine and around the Krim. They will also get tens of millions of new citizens, lessening the problems of a shrinking number of people available inside Russia significantly. There also are significant parts of former USSR production in Ukraine, which will all go to Russia. They will also own even more of the prime agricultural lands of Eastern Europe, which at least so far seems to suffer less than Western Europe (look at the heat maps of this summer) under climate change so it may become even more valuable than it already is. The land is some prime real estate - unlike Siberia, Ukraine is much better, you can't look at the map and think "it does not add all that much to Russia" because the value of Ukraine lands is much higher.
I have no idea how you get this idea. Giving up Ukraine is really, really massive in its long term consequences, greatly strengthening Russia directly as well as showing them that the means they use actually work. This would be a gigantic loss for the West.
