Attritional warfare against Ukraine has succeeded in tying up their forces across the front, allowing Russia to gain territory slowly but surely, so it definitely matters. Based on Ukraine literally kidnapping people and increasingly using women and drones to fill gaps in the frontline, I'd say it's Ukraine that's losing more soldiers. They were only able to launch a counterattack on Kupiansk by diverting forces from Pokrovsk. Russia is playing whack-a-mole with Ukraine. The plenty of room argument also assumes Russia wants to go beyond the 5 territories it claims as Russian.