It may not be theory alone. For me I can use different types of blades for different amounts of time comfortably. Its a range but for this I'll stay at the lowest end of that range. If I figure up the amount of blades I can get for $20 with the amount of shaves I can get from each type of blade I get something like this. $20 DE blade (Kai) 150 shaves ($0.13 a shave) $20 injector blades (Schick) 112 shaves ($0.18 a shave) $20 SE blades (GEM) 80 shaves ($0.25 a shave) $20 cartridge (Gillette fusion) 20 shaves ($1 a shave) Sure we spend more on razors now but we save so much on blades that most of our razor purchases are bought using our savings. Some people wetshave using carts and disposables and some use canned foam with the vintage types of razors so I couldn't count soaps brushes and aftershaves as part of this switch. Straights are a different beast. Sure one blade can last multiple lifetimes but strops, Hones. . . .
Then we have guys like @Bama Samurai who can use 2 DE blades a month. That same $20 could get him 750 shaves or 2 years and 20 days as long as one of those isn't a leap year.
Or some kind of sensitivity. I can't use Arko becuase it makes my face red and angry, and it kind of burns.
What type of bowl is that? There are many lovely styles- Polish, Talvera, Turkish. Never seen that style
I have a Feather folding shavette. I hold it like a Kamisori. Wish I had gotten the staright handle like yours.