If a thumb hole was so terrific, you'd see them on more whistles. I'm just saying...
Yes, that's basically it.
The C thumbhole is mostly a solution in search of a problem. The only players I've personally seen using them are very beginners, who ended up taping the holes as their playing improved.
Tradition, schmadition. Players always have a choice. Hole technology has been sufficiently advanced and widely available for the past 150 years that any whistle player who wanted a thumbhole could have it. Today there'd be a lively aftermarket, especially among top players, for retrofitting thumbholes to older whistles to correct the design flaw. In some other universe, maybe.
To experienced players, the thumbhole simply adds one more alternate fingering to the already ample inventory of C-nat fingerings: half-hole, oxxooo, oxxxox, oxxxxo (high), xooxxo (high), etc. Talk about overkill. Especially for one note, whose intonation in trad playing is variable anyway.
I've tried the thumbhole a few times, so I know what it feels like and how it works. If I happened to have a whistle with a C-nat hole I might use it occasionally if the fingering ergonomics of a particular phrase suggested it. But as it is, after a number of years playing many different whistles, the thought "Gee, I sure wish I had a C-nat thumbhole" has crossed my mind ... well, never.