That's a huge hindsight bias. Even the most optimistic scenarios from various secret services / think tanks predicted only weeks / few months of resistance at the most. If you read Ukrainian accounts, most weren't very optimistic either. You can also read up how many commanders in the south defected - having more such defectors could flip the war quite quickly. The question of whether to defect or not (or simply run) is something which many did on the spot based on their personal circumstances and outlook, it's extremely difficult to predict.