There is never a end to it. In the 18th century, after the work of leibnitz , newton , euler we ended up with the mathematical tools to describe the physics of bodies in motion really well using differential equations such as Newton laws. Likewise in fluid mechanics (bernouli) and celestial mechanics (newton and others ).
At this time physicists thought they could finally describe the world and they were done. They kept applying this approach in the 19th century with thermodynamics ( clausius, thomson, gibbs, kelvin ) magneto statics, electrostatics, etc.. ( maxwell, faraday, etc.. ) .
It gradually became obvious that this was not the end of it. Einstein, swinger and others opened up a whole new area of physics when they showed that the classical description was just an approximation of a more complete theory ( general relativity, quantum mechanics, etc.. ) . The early part of the 20th century was another golden age.
Today, we are going further, refining our knowledge. The current big hurdle is figuribg out hiw to have a unified description of all laws of nature, unifying gravity with strong and weak interactions, as well as quantum electrodynamics.
LHC at cern is one tool we are using to explore that. There is also very active research on gravitational waves ( ligo experiment) which just got a nobel prize ( kip thorne).
It is far from being a dead field. It just is becoming more specialized. Newton was able to make contributions in several areas of physics as well as mathematics. Nowadays, you specialize in one area because the body of knowledge you have to absorb before making meaningful contributions is vast.
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