Example: Google is still a tech company. As much as some might view search as "solved" (just like it was when Google was founded I might add), search continues to get better. Tech still plays a significant role in the actual core business (ie advertising) too.
I once again return to the Steve Jobs take on why Xerox failed [1]. An effective monopoly devalued the tech such that Xerox became a sales and finance company.
So, returning to Netflix. I agree Netflix isn't a tech company anymore because Netflix ultimately is about serving several thousand VODs and that problem is solved. Now Netflix's core business is about making content, licensing content and acquiring and retaining customers.
Obviously doing this (or anything) at scale is a lot harder than it sounds but at the end of the day Youtube is monumentally more technically challenging than Netflix due to the number of videos, the number of viewers, the advertising and live streaming.
"Tech company" is still a useful classifier IMHO.