Of course, in-universe, the Enterprise was, by the end of Kirk's first 5-year mission, several decades old (how many decades depends on how much time had passed between Robert April taking command out of the shipyard, and Christopher Pike going to Talos IV). There are only so many technology refreshes that can be done out in the field.
Of course Roddenberry, Coon, Justman, Solow, Jeffries, et al. didn't set out to make TOS look like it was done in the 1960s; that was a natural consequence of the fact that it was done in the 1960s. Just as the fact that TMP looked like it had been done at the end of the 1970s because it was done at the end of the 1970s.
Of course Roddenberry, Coon, Justman, Solow, Jeffries, et al. didn't set out to make TOS look like it was done in the 1960s; that was a natural consequence of the fact that it was done in the 1960s. Just as the fact that TMP looked like it had been done at the end of the 1970s because it was done at the end of the 1970s.