And what would Europe heat itself with? Farts?
It's not like your leaders care about Ukraine and Crimea THAT much. Otherwise, they would have done it 4 years ago, if it was that easy.
And yeah, talking about gas. The construction of Nord Stream 2 (a gas pipeline from Russia to Germany) has started in Finland. Despite of some countries (including Poland, Ukraine (that wants Russian gas to transit through its territory despite of having a 'war' with Russia) and, unsurprisingly, USA) opposing it. That says a lot about how Western Europe tries to 'isolate' Russia.
There is an opinion that this Skripals story has something to do with Uncle Sam's attempts to frustrate this project. Simply - when you have a deal with Russia, you deal with evil, bloody, ex-KGB dictator Putin who poisons random people in Britain (and can do so anywhere) with an incredibly toxic nerve agent. A literal new Hitler.
As for the sanctions - if you don't remember, they were already imposed just after the incident (you don't need any investigation when blaming Russia, do you?). When the US want sanctions, they'll impose them anytime they want, and will find an appropriate reason (like, invent the 'US elections meddling'). Even if RF left Crimea and Syria, I'm sure that the sanctions wouldn't be lifted. The USA would demand Russia to give the Kuril islands to Japan, and impose more sanctions for hacking a power station in Tajikistan. Once you show the weakness, you'll be pressured to the end. That's how politics work.