"Why should I learn the language, syntax, ecosystem etc... When I can just Google my problem"
It's amazing we live in an age where anyone's programming problem can be solved through a simple query to the Google...
You seem to be speaking of this behaviour in a way that implies negativity. Of course it is bad is a Sr. Dev truly can't write any original code, but there's nothing wrong with the vast majority of ones codebase being snippets of solutions from the internet. As a matter of fact this method likely saves the company that employs the developer a good deal of time and therefore, money,
Even Joel Spolsky (creator of SO) has brought up some negative impacts of StackOverflow (on his old podcast at some point), saying nobody needs to learn anything anymore, just jump into the code and google the error messages.
I'm guilty of this too, I just think it's a shame that Googling is preferable over learning...
I feel the average junior developer’s, or even senior developer’s, for that matter, code was a lot worse than the average SO response, which has now become the baseline for all types of devs, so their code is at worst as bad.
When I look for an answer, I do look to Google and/or stackoverflow, but then I immediately turn to figuring out how to find the "real" answer.
Also, a lot of times, Google's approach to identifying the single answer to your question means it masks any similar question, making it severely handicapped in answering anything difficult. But if it wasn't a difficult question, I wouldn't be asking!
You cannot develop an app, evaluate different solutions, choose an architecture, address performance problems, etc, via StackOverflow. StackOverflow is for specific problems; it can help you sort out some errors, because most of them aren't exactly coding problems.
A senior engineer's job isn't just to make an app compile, so you cannot reach this position just by searching SO.
This says more about the company's definition of "senior engineer" than about StackOverflow.
(Nothing wrong with StackOverflow, and knowing how to search for answers there is definitely a valuable skill, but it's not enough to become a senior engineer)