Pathfinder 2e's playtest did effectively release an entire Player's Handbook, along with several adventures, to properly test out the system. Granted, those were paid which is a different model, but it's not impossible for a test.
My understanding of the D&D Next playtest was that it was a gradual releasing of vertical slices of the rules, enabling more through testing.
Cyberpunk Red's playtest version was released as a small handbook containing a subsection of the rules allowing them to be tested and games ran.
It is not impossible for WoTC to adopt a vertical slice approach, and split combat rules and changes to monster rules, character creation including a subjection of classes, exploration, and changes to spells into different playtest packets (along with other categories I have forgotten) that would be more modular and easier to test in a closed system way without requiring an entirely new PHB to be produced. Right now, we have a big mixture of different rules affecting different parts of the game mixed in together with unclear results on each other - a horizontal slice. I feel that's going to have a negative impact on the feedback received and the game design.