Web Assembly has only really been properly supported in (all) modern web browsers since late 2017, and even still its support is pretty rudimentary. Any big company is going to be apprehensive about putting it into production for an important product like Runescape at this point. The company I work for is apprehensive about putting nodejs into production and it's been around for 10 years!
Seems a bit silly but given the (potential) risk to the business, I understand the concern surrounding adolescent technology.
https://blog.mozilla.org/blog/2017/1...y-in-browsers/