Reading Kernel memory (Meltdown attack) is extra bad but regular user processes being able to read each other's memory (Spectre attack) is also very bad and not solvable by isolating the kernel.
Im less worried about my steam client reading my chat cache than something inside my web browser reading the keys that encrypt my home directory. Short of abandoning all sharing, the least we can do is isolate kernel cache.
That depends on what you are chatting with. My chatlog would be very interesting to our competitors. The key that encrypts my home directory isn't useful because the firewall blocks your access to my home directory (that a different layer of security).