This kind of policy doesn't last very long: did you know that the USSR collapsed because it spent too much money on the military?
The USSR overspent on their military.
Right now, Russia's military spending is less than what USSR spent in terms of percentage to GDP (6.3-8% vs estimated 15+%), and a large chunk of that is due to fighting a war, which will reduce to a significantly manageable level once done.
If their govt revenue and spending grows at current pace, and war ends in the next 2 years, resulting in a budget reduction from 13.5T rub to just 8T rub, they will maintain a 10T rub surplus.
If they still keep their defense spending up at 13.5T, they get to maintain a surplus of around 2T in 2027. If post-war budget drops to 10T in 2027 and its growth sustains alongside growth in revenue, then they will maintain a surplus of up to 6-8T progressively by 2030 alongside a 13T defense budget in 2030, which they can save for a new war.
If they spend 7-8T on procurement, it will be the same as what all of NATO spends on procurement. And they can do it with a massive revenue surplus all the way up to 2035-40. That's around 150T in surplus at least. Oil prices may start dropping after 2035, but Russia's oil industry is the cheapest to maintain, and India and China can sustain as markets for the longest due to compatible refineries, so it will also be the last to go.
It's all more than sustainable until 2045.