Your alternatives for supplying gas to Europe couldn't meet demand. Cost would obviously increase cause of high demand with little supply. With high prices and little supply, Every country would want to maximize its oil and gas reserve use so would your alternatives sources.
Russia supplies around 200 billion cubic meters of gas to Europe every year (40% of the total demand). If you take the supplies from the alternative sources (US LNG, Venezuela.etc), the spare capacity is less than 10% of the amount provided by Russia. On top of that, new regassification and LNG terminals take at least 5-6 years to build. To complicate things further, the Biden administration has taken many policies that negatively impact the shale oil and gas producers within the United States, ever since he took office in 2021 January.
Russia supplied this amount of gas to European countries earlier. Now the situation is rapidly changing not in favor of Russia because of the aggressive war unleashed by it and the desire to seize Ukraine. If just a week ago some European states were not ready to impose an embargo on supplies of oil, coal and gas from Russia, then after the publication of those terrible atrocities that the Russian invaders perpetrated against the civilian population in the Kiev region, Europe is almost ready to impose such an embargo even to the detriment of own interests. This is facilitated by the end of the heating season, and the previously proclaimed reorientation to "green" energy from alternative sources as a measure against a sharp climate change. European countries have realized that they need to get rid of their energy dependence on Russia, and therefore Russia will gradually lose its positions in this regard.