It stops being new when something newer comes along in my books and so far nothing newer has been seen, everything is a poor copy of bitcoin. On top of that if we consider the adoption, bitcoin's is still less than 1%. One way for bitcoin to stop being "new technology" is actually being adopted and used.
Another view is that Bitcoin is changing and improving, in other words not only the technology is new but also new things keep coming out: Script hash, locktime contracts, SegWit, Taproot, Lightning Network, ...
That's one meaning of the word "new", but other meanings are "fresh" and "recent", and Bitcoin can hardly be called such. And as for adoption, no one has any idea just how well adopted Bitcoin going to be, and there's no good method for predicting it.
Sure, Bitcoin keeps improving, just like everything else, but fundamentals are staying the same.