It’s not a theory. It’s a fact. It’s very simple. We didn’t start numbering with the year 0. We started with the year 1. The completion of that decade was the year 10. The decade begins with the year that ends in 1, not 0.
And, yes, I would call the year 1950 part of the 1940s decade because that is exactly what it was.
I appreciate your consistency but....please give my regards to your allies on this point in the phone booth. You remind me of a friend of mine who always corrected me when I didn't pronounce El Salvador in the proper Spanish accent.