Well you do have to recall there was a good 25 years between the shows. Budget on TOS was non existent. All the following series had multi million dollar budgets which covered writing and sets.
The great thing about TOS was the relationship between the three friends. And yeah, some of it was silly. It was geared towards kids mostly. But it did bring up a number of issues and was able to push TV in to some complex issues of the time. Issues today we might think silly. And it's still had the most beautiful ship designs. STNG's first Enterprise was just fat and ugly. Though the E did the name justice.