Note that with Kotlin, JetBrains created it initially for working on their IntelliJ IDE and are porting their Java codebase to use Kotlin. So that is a good indicator of long-term sustainability.
As for Valhalla, it remains to be seen how it's usable from Java, let alone anything else. But Kotlin already has a form of value type (inline classes) so perhaps it'll be an extension of that.
One cannot just change their semantics after the fact.
case class Foo(x: Int) extends AnyVal {
def square: Int = x * x
}
val bar: Foo = Foo(5)
println(bar.square) // => 25
object Foo {
def square(x: Int): Int = x * x
}
val bar: Int = 5
println(Foo.square(bar)) // => 25
inline class { ... ]
There is also precedence for this, Scala 2.12 changed the encoding of lambdas from anonymous classes to Java 8's invokedynamic.