Spelling is only the way we record a language
Duncan Niven makes two major errors.
First, he mistakes spelling for the language. The language is words, their meanings and etymology, their pronunciation, grammar and syntax - and includes spelling. The language is a natural phenomenon, spelling an artificial creation to record it visually.
Second, he suggests that rationalising and upgrading spelling is "dumbing down". No doubt in the same way that decimalising currency is dum(m)ing down, changing from Roman to Hindu-Arabic numerals is dumming down, progressing from penny-farthing bicycles to multi-geared mountain bikes is dumming down!
Most progressive changes have suffered such charges before eventually being achieved and accepted. Upgrading English spelling needs to be one of them.
Christchurch, New Zealand