An interesting topic. I also know Ehrlich Yidden that hold this Minhug, however, they (probably) weren’t so careful if mistakenly said A Gutten Shabbos on a Friday afternoon, to keep Kidushas Shabbos & Isur Melocha from then on.
I also understand “old man” point, but should have been brought out with more respect towards Torah.
Basicaly said, this is a Hidur / Chumrah in Kidushas Shabbos.
Btw,in a Neder there can be Divorim Sh’blev, if it’s said out in a Loshon of Neder but wasn’t meant so.