Gijs, I meant only (as Joachim guessed) that I thought 8x was uncommon back in the 1940s. When growing up in the 1960s I only had or saw 7, 10, 15x. Unfortunately we weren't birders, I only came to it later in life. I'm enjoying the stories of those who started early. What caught your attention? I think it wasn't only birds that I wasn't noticing...
I wasn't around then, but the impression I get is that 7x was more popular in the States back in the day, whereas 8x might have been more popular in Europe. 8x30 certainly has long been popular amongst European manufacturers. Allbinos has an informative page covering Zeiss's 8x30 Deltrintem and variants that were first made in the 1920s, and Leitz's 8x30 porro first appeared, it would seem, in 1927. I'm not so familiar with 8x40s, but these were definitely produced by Zeiss (and probably by other European manufacturers) and, post-war, in Japan. Later on there were the classic 8x40 and 8x32 Leitz Trinovid roofs and the 8x30 Dialyt. Peterson's recommendation of 8x matches my own (meager by comparison, I hastily add!) experience very well.
As to whether "field glasses" are optional - well as the replies above illustrate, for some types of birding, they are, if one's fieldcraft is up to par. Mine isn't, and being able to take a closer look at the birds greatly enhances my enjoyment of the day, so binoculars are most definitely essential for me. The birding I do most often can't be done without binoculars. The building that one pair flies from is over one kilometre from my vantage point.