Russia has a lot of people, Ukrain is getting drained, there is no denying that. Sure Russia has made errors, so has Ukraine. Who can afford errors and deaths more? The larger military force. Vietnam showed outwardly a different story. But they also had other advantages Ukraine does not. And not to forget, Ukraine has been and remains a deeply corruption and morally depraved relative of Russia. Run by criminals and human traffickers. In the years prior to the invasion, they were fighting pretty dirty too, shooting down Airlines then denying it. Their fighting corps being seasoned with NeoNazis, being pretty disgusting to certain groups living in Ukraine etc. But of course nobody wants to remember anything prior to the invasion. And no, this isn’t pro Russian, just more realistic.
Hardly. A lot more reasons than can be squeezed into one. Taiwan needs to prepare to defend against China is a reason, but is also just one of many.