> A drawn out conflict favors Russia
No, it doesn't, except in the sense that the conflict would have to shift to favoring Russia for it to be plausible for it to be “drawn out.”
Russia has already seen casualties that are probably on the order of those the (much larger) Soviet Union saw in its decade long war in Afghanistan, and significantly higher among senior officers—and that wars toll was a significant factor in the collapse of the Soviet Union. And the toll on equipment which they have basically no way to replenish has been high as well.
> Last time I checked Russia had a 10/1 advantage in heavy artillery
Don't know when you checked, but Ukraine has been getting a flood of artillery from the West, with longer range guns than the Russians have.
> has not mobilized its army
They haven't done a formal mass conscription, instead doing a backdoor mobilization, but that's kind of irrelevant since they are running out of first-tier combat-ready equipment, pulling older stuff out of stores to send to the front.
> fighting with 1/5 of its available peacetime forces
Maybe in total manpower. One of the many reasons for the high casualties in senior officers (besides the inability to delegate in land forces) is that their combat-capable pilots are a narrow corps that is senior-officer heavy, and they have a similar issue with other critical specialty forces, and those can't be replenished on any reasonable timescale.
> has suffered massive casualties among its most capable forces.
Russia has suffered massive casualties both overall and among its most capable forces, and expended a lot of its most capable equipment, and no one is rushing them replacements.