No sequels were necessary
This is strictly my opinion. I'm sure there are a few others in agreement. I've seen the original countless times and to me it works best as a stand alone film because its a coming of age story.
Daniel, while not the most popular, is fleshed out. Mr Miyagi's character is well developed since Pat Morita had gotten nominated as best supporting actor. The Cobra Kai were the perfect antagonist gang to Daniel with a girl caught in the middle. All of this leads to one of the most memorable fights at the end in the tournament battle.
Daniel defeats his bullies in order. He gains the respect of their leader and Mr. Miyagi's philosophy comes full circle. Daniel believes in himself and captures the confidence to do what he's able to do. This is a message to viewers and it's driven home nicely.
The sequels are just a way to cash in on the success of its predecessor. It was unneeded and they all got worse as the series went on, with Part 3 as the weakest of the bunch with such a lame close to end the franchise.
For those of you who disagree with me and feel the sequels were needed please speak your truth and explain.