Notice who is pushing the narrative of Russia bogged down - journalists, politicians and their nominees, economists, and people with an active career in the military. There are also some dissidents in the West pushing the opposite narrative - ex-marine, retired intelligence officer, weapons inspector, colonel from a neutral country. Which group knows more about how a modern war is waged? Which group has a better track record so far? Which group openly admits they're releasing unverified claims in order to shape the public opinion?
> How much would it cost to continuously occupy a country of 48 million largely hostile people
If Russia limits the territorial acquisition to Russian-speaking regions, they'll be fine. Minorities are almost always strongly in support of unification with their nation-state. Some historical examples - 77% of Sudeten Germans voted for Nazi Sudetendeutsche Partei. 99.7% of Austrians voted for unification with Germany in 1938 referendum. 95.6% of Crimeans voted for unification with Russia. 95.7% of Cypriots voted for unification with Greece in 1950
> How much would it cost to continuously occupy a country of 48 million largely hostile people
If Russia limits the territorial acquisition to Russian-speaking regions, they'll be fine. Minorities are almost always strongly in support of unification with their nation-state. Some historical examples - 77% of Sudeten Germans voted for Nazi Sudetendeutsche Partei. 99.7% of Austrians voted for unification with Germany in 1938 referendum. 95.6% of Crimeans voted for unification with Russia. 95.7% of Cypriots voted for unification with Greece in 1950