They did release all armies at once. Every army had a playable list at launch.
They didn't release miniatures for all armies at once. That would have been impossible. That would have been, by far, the biggest miniature release of all time. During a year where GW is struggling with production capacity, releasing hundreds of models and kits all at once would just be impossible. Even in a good year it would have brought every other product line screeching to a halt. Its ridiculous to think this was even an option.
I think so far their release plan has bene fine. we already have 3 full model lines released, with a 4th coming very soon. Its not very hard at all to get models for most other armies, even without resorting to 3d printing or proxying. I pretty easily put together a full Dark Elf army with a trip to my local shop and a few ebay purchases. My group has full Dwarf, Empire, Lizardmen, Beasts, High Elves and Skaven armies, in addition to people who bought the already available Tomb Kings, Brets and Orcs& Goblins. No one had much trouble getting what the needed, except for the guys trying to do original Chaos Dwarves. And even he's up to 1000 points.