Going for 42mm, is something I would also like to remain limited in terms of number of references as well.
However, is it really a fashion trend? Like aesthetcial trends that evolve every x years? Or is it a more objective and long run evolution?
If we look back to the 1930's - 1950's, the watches were much smaller (from 30mm to 34mm let's say, on average). More recently they were let's say around 36 to 40 (let's remain on dressier watches and let's not talk about sporty models which can bear 40 easily).
Then, you see that the standard has evolved very significantly and that it was not a fashion. It is only what we are used to seeing (and more practical to read).
About your 3940 example: yes, people tend to like it. However, I think that it touches more people that are collecting watches and who we can see mainly on forums. Thus, the people sharing their 3940 on forums don't necessarily represent the larger part of buyers, who is going younger and where new generations replace former ones.
So, yes the 3940 si successful today for collectors and forumners but what does it represent among the new generations? My personal opinion is that they don't buy really 3940 but bigger watches (and with a different asthetics too). Thus most people prefer bigger sizes today than the 3940's.
That's why I don't think it is a "timeless" issue (except for above 43mm of course) concerning 42mm watches as it can remain "in" permanently.
In the end, is it a "timeless" issue? Around 40 (for 3 hands) or 42 (for more complicated) is fashion? Or just that you don't want to see 42 because you feel this is "hurting" your own taste and preferences?
I think it is more the latter and please know that I totally understand that and I can feel the same myself (I also find the 42mm too big, for myself). But I think it is a very nice offer and worthy of Patek, without jeopardizing the brand's image.
The thing is that the brand should not be ahead of the others and be bigger than others.
Cheers MB.