They can. And they will.
They'll fight with sticks if they have to. The idea that they'll rollover and die is just preposterous.
Also, while stalemated, Ukraine has been able to dramatically increase domestic production. All those FPV drones are made in Ukraine. And recently companies like Roshel of Canada and Rheinmetall of Germany have started building manufacturing facilities in Western Ukraine that will pump out arms.
Meanwhile Russia is so desperate they are paying millions of rubles in recruiting bonuses, imposing indefinite contract renewals, all while not meeting recruiting goals and shutting down prisons after sending all those prisoners to Ukraine. They are draining pension funds to pay for this war, along with all kinds of other reserves. If this war continues for a few more years like this, they might actually need an IMF bailout. And even if it doesn't, the country is irreversibly screwed. They blew that massive Soviet stockpile, hundreds of thousands of lives and trillions of rubles on gaining like 20% of Ukraine. Those gains, even if cemented, will never pay off the economic and social cost. This is why Putin is going all out to get Trump to impose a settlement of any kind.