I'll risk sounding insistent, but ... You introduce complexity adding -I%, since it's unnecessary. Furthermore, it leads to a wrong result since the final filenames won't have the extension replaced, as expected, but appended.
Once again perhaps sounding insistent, but ... the basic philosophy of parallel is to process 1 argument at a time: when you introduce --max-args 1 you divert attention from its basic way of working. Someone working with parallel for the first time will be misled to think that it's always necessary to include --max-args. As you said before, it's best to avoid introducing unnecessary complexity in a tutorial, which you do introducing unnecessary options.
I'll risk sounding insistent, but ... You introduce complexity adding -I%, since it's unnecessary. Furthermore, it leads to a wrong result since the final filenames won't have the extension replaced, as expected, but appended.