Yes, it could be considered a “clutter” however when it was a stated game limitation that “trees cannot have more than 5 lines” I think this is a good solution for ever decreasing space.
Especially if they try to organize trees by the country (see Hungarian and South African sub-lines) which will leave space for the “original” nation to get more stuff.
As well as that, foldering should only be done when its similar vehicles, and not to fluff trees. Adding in folders to literally everything does nothing but increase the use space of vehicles that may or may not be similar depending on the BR, like when the BMP-1 was foldered with the Marder 1A1(-) instead of said Marder being foldered with the Marder 1A3 right after it.
Also this seems incredibly selfish. Being against an idea for improving the game unless they make an “optional” version just because you want your stuff to stay the way it is currently is very short sided.
Besides, the original post specifically mentions foldering nations.
If you want a specific vehicle but don’t want to switch the nation viewed, you have the search bar.