The article states that DoH will become on-by-default in september.
In general, I find it highly unlikely that it will stay off-by-default forever, because there is no way to have any meaningful adoption of it as an expert-only feature.
Nothing in Mozilla’s communication even hints at DoH becoming default on any time in the nearer future. I’m certain Mozilla would like encrypted DNS by default, but not at all cost. It will probably land as a generally available feature in September, but still default to off and still be behind about:config. There are many expert features hidden in about:config that might never become default or only after a substantial shakedown period. Third party isolation, for example. So it would not entirely surprise me if DoH would remain an expert feature for a long time. And I’m certain it won’t become default on without a very clear config UI. Messing with name resolution has massive impact on a lot of setups.
In general, I find it highly unlikely that it will stay off-by-default forever, because there is no way to have any meaningful adoption of it as an expert-only feature.