I think the German blitzkrieg solved the first instance of a protracted front line but when that ran out of steam the fronts were not as fluid. If Germany had not of invaded Russia then perhaps the Allied counterattack may have bogged down.
i do agree with you that war has changed and its like a "fighting war" gets like the Somme was. But winning a war is also harder as an army may control the streets and the government but the war moves to more Terrorist type activities. Classic example was France under Nazi control. Modern example is Afghanistan. That's a type of stalemate.
Let's be realistic about this, the Russians have culminated and are a spent force, they got behind the curve with mobilization and took massive casualties. It has been a killing ground of Russians mobiks at a ratio of at least 5:1 for the Ukrainians, but it has been an attritional slog and not the war the Ukrainians want to fight, even if it broke the back of the Russian army with stupid mass assaults on defensive positions in the middle of mud season, only a moron attacks in mud season! While they were receiving western arms during mud season training and preparing on sandy dry ground, it is mostly older territorial veterans who are on the fronts in defensive positions supported by regular army troops. The Ukrainians have been constantly training new units in Europe and at home and have been replacing loses and creating new forces and have the equipment and armor for them.
Russian morale is rock bottom the troops are not even trained, poorly armed, equipped and led. They are also facing an acute manpower shortage and it appears that Vlad is sleep walking into disaster, some around him say Vlad has gone mad. Expert opinion has the Russians ejected from mainland Ukraine by summer, driven to the borders and some expect them to take Crimea by fall or by next summer. I think much depends on when Joe wants to spring the trap and give them long range weapons.
Once the fight begins and Vlad pours as much as he's got into Ukraine the trap will spring shut and long-range weapons will be provided to take out the rail bridges and road bridges leading into Ukraine inside Russia. It would only amount to a half dozen rail bridges inside Russia. Since they can't operate more than 50km from their rail heads, those railheads would be well inside Russia. They would not be able to get new supplies or fuel to their troops in Ukraine or new troops and equipment there either. The troops in Ukraine will have to deal with encircling blitzkrieg operations chopping them in half and then into smaller pieces. It will be fast, more humane and cost less causalities on both sides. It will be a lot like the early days of the German Soviet war when the Germans encircled hundreds of thousands, almost a repeat. Unlike the Soviet Union, the Russians won't bounce back, but will stay behind their borders and lick their wounded arseholes for a decade as their economy goes completely to shit and their empire dissolves.
The objective is to destroy Russian military power and their ability to project power onto their smaller neighbors and eventually force Vlad out a high window. Blinken and Austin said as much quite a while back at a news conference, before Joe told them to STFU! If they gave the Ukrainians long range weapons, they would be forced to use them to save lives by cutting the rail bridges, this gives them a way out, just in time delivery it's called. Smart Ukrainians know that the only way they will have long term security is to utterly destroy the Russian army and Vald has 92% of it in Ukraine now, so there's not much more he can throw in, even mobiks for now. It might suck for the troops and civilians, but the top brass would know and agree, probably most would. It would also extract the last measure of revenge and retribution and the large numbers of prisoners should get Ukrainian hostages, kids and soldiers back home.
