Ukraine needs a steady supply of modern weapons and the freedom to use them on Russian territory. That way they can continue dismantling the Russian military and minimize casualties. The Russians are pulling a lot of equipment from storage and are refurbishing it. But those storages won't last forever. Also, Russia may have a large population (so does Ukraine), but pretty much everybody willing to fight is already fighting. Also economic hardships are only going to increase. In other words, political instability in Russia is increasing and with it Putin's ability to mobilize troops. Don't forget, Putin can pull out at any time without loosing his power. For Ukraine it's about survival.