From the dev diary, just above said change:
An issue in EU4 that we've long recognised is that conquest is almost always a good idea: you are able to immediately get a financial benefit from land, buff up your own forcelimit, size, trading potential, while at the same time denying your foes that land. We've been wanting to change this so that one has to consider what they conquer with a bit more forethought and with that we turn to your States.
The motivation behind both the corruption from large expansion and missionaries/culture change in states only is all here: to prevent taking more land from simply always being the most powerful thing for you to do for your nation. If you see land of a different culture group, different religion and undesirable for making a state and core, then you should have to consider if making a direct land grab is the best move, as opposed to subjugating it, forcing trade power from the owner, establishing a powerful ally there, investing in tolerance of heretics/heathens etc.
Now, some people may not agree with the motivation behind the changes, and that's fine, but not agreeing with them doesn't mean they're not there. Whenever we do so called anti blobbing changes like these, the feedback is always prominent, and we do listen and communicate with the community.
Some changes are simply unpopular, that goes for most walks of life. We didn't put these changes in thinking "people are gonna love this", we did it to break the stale strategy of direct conquest always being best.