Usually there is a delay before they switch lines, when they tested terminus for example, they tryed a couple of different things and the first public test was made with Stark if I remember correctly.
Ultimatly Terminus was not suited for everyone, some people loved it because if the strength and the speed at which they got results, but for some people (like people who weren’t that used to running subliminal), it was almost impossible to run.
Terminus didn’t replace the final build, and they made the build avaliable on the Q store for advanced users and explained what it is in the FAQ, while still considering Q the main branch of product because results were more stable.
It’s not like they are just creating new prototype and switch the whole line of product to that new prototype instantly hahaha. If people react greatly to the prototype and it can be slowly tested on multiple subliminal and different people, then depending on the feedback, the decision is made.