Balaji,
To a certain extent what you told about NT's films after 1975 is true and whatever good films happened could not be compared to the classics of 50's,60's & early 70's. The main reason behind this development was the remake culture that set into Tamil Filmdom. Earlier there were story writers and directors who were straining every nerve to write stories for NT and his acting talent. But during the mid 70's, the trend in Tamil began to change and NT accordingly had to do films which catered to the taste of the audience of that period. Even before this NT had a strategic change in the late 60's as for as his roles were concerned. Before going into this, let me tell that point also.
From 1952 to 1965/66, NT's fans were considered to be of soft nature consisting mainly of educated,middle class people. But slowely a group from the fans was emerging aggressively and they wanted NT to give a fitting reply to his detractors. That was about the criticism that NT can't fight,do action roles etc. The fight between the two Thilagams was growing and since both happened to be on the opposite side of each other in political camps too, the rivalry between the two fans' grew manifold. After a certain stage this group became vociferous and since it consisted of mainly youngsters, NT had to give in. There comes Balajee and his Sujatha cine arts.NT having not done stunt scenes till that time (barring a few),though he was 112 films old, tried his hand with "Thangai" (1967). Since there were not many writers to script an action film for NT, Balajee bought the rights of a Telugu movie and ACT (his first film with NT) directed the same. It went on to become a hit and with Balajee coming out with more action oriented films like En Thambi & Thirudan, other producer/directors also started making movies of NT in that genre.
So coming back to 1975, when again he needed continious hits to suit the changing trend, the writer/directors choose the safe path (ie) remake. So when you borrow something to suit the giant, there were many hitches and it showed in the quality.
Regards
Let us discuss films after 1975 yearwise.