The idea of running some sort of an insurgency has been floated around a lot. I agree that it's a dumb idea and I imagine it will backfire spectacularly because Ukrainian nationalists will be resentful of the west when the war is lost. They will blame the west for not full supporting Ukraine and for using them as a battering ram against Russia. Europe might end up seeing a sharp rise in terrorism in the near future. Also worth remembering that a lot of the weapons sent to Ukraine ended up on the black market, and likely in the hands of far right cells across Europe.