Because I'm old I'm going to bore you with a story.
About 20 years ago (or more) I bought a friend of mine a TOS cast t-shirt because he loved TNG. Loving my gift
he took it away on a holiday to climb the Himalayas where he would be camping and staying in huts without running water and electricity.
He came back and he told me that he was amazed how many people recognised the people on the shirt. In places without TV or movie theatres where they didn't speak English, people would come up to him and say 'Kirk', 'Spock'.
So you claim they're not icons but a lot of people around the world recognise them.
Actually, it is a common story. In my travels, the image of Shatner as Kirk / Nimoy as Spock were and remain instantly familiar--unlike the majority of random actors who are in and out of a role (even if successful), with limited effects & for some reason, the personalities and/or vibe
of the performers were not so locked into the characters as in the TOS case.
The only other rare cases I can think of where the actor so defines a role that all others to follow never matched the effect/association: Karloff as Frankenstein or Garland as The Wizard of
Oz's Dorothy or Lugosi as Dracula. Rare and historic company, as all have had others take on the characters on film and TV (some famous, such as Lee's Dracula), but blink, and you missed most who followed.
That is the measure of the iconoc character tied to and eqully iconic performer: anyone else in the role would not be at the right place/right time of their lives to influence/define a character in a way which touched people (from various walks of life) so much, that it had been impossible to replace or match the performer/character association.