They wouldn't, simple as that, they're short of troops, and short on equipment, they can't even keep up with replacing the kit they lose every day. There's no way they could commit to taking on another battle for years, possibly decades, they can't even get past Avdiivka, or Bahkmut, and they were pretty much at the door of both in 2014, never mind 2022.
NATO isn't going to stop providing weapons either, and anyone thinking that NATO isn't already at the Ukraine border doesn't know what they're talking about, they wouldn't get 1 KM into Poland, Finland, or the other Baltic States at the moment.