I wonder how the demand for gas will be reduced to this percentage (15 percent is not a small percentage),
Possibly after a bunch of gas hungry industries around Europe shut down and millions lost their jobs, the demand could decrease by 15%. 
France, Germany or Spain can easily afford alternative source of power but what about less wealthy EU nations. Diversifying their energy source would be difficult because most of them cannot afford it.
It is not always about affordability, it is also about quality and availability.For example the oil refineries that worked with Russia oil can not work with another oil like the one coming from US because they are different (light and heavy). So they basically have to rebuild the refinery if they want to change their source.
There are other alternatives that are similar, like Iranian oil that is the same exact thing as Russia oil but they face the challenge of availability since Iran isn't going to sell oil to Europe more than this.
Same with gas. The qualities are not the same. For example from what I gather the US gas is among the lowest quality gases alongside UK gas. So even when they import some LNG from US it still is low quality. Not to mention that US already significantly reduced its LNG export after the fake explosion in their LNG facilities.