I've heard the same thing, that 7bbl is generally the smallest you want to go for a sustainable business. As you pointed out, there's definitely a better margin on selling your beer directly vs wholesale which can help make a smaller setup more reasonable even if it's not as efficient as a larger 10bbl system. However, I didn't have time to play bartender, while holding down a full time job, and launching a brewery. Or time to distribute that much beer for that matter.
My plan was far from traditional though. Since I was going to brew in my home, I would have no rent. I had no additional employees to pay since I was the brewer, delivery driver, and sales manager. I had an agreement to buy all of my ingredients directly from a local homebrewing store (only possible due to my small system!) which saved a LOT on shipping costs vs getting ingredients from wholesalers. Shipping is a larger expense than most would guess. Plus there was no 5 year contract like all the hops companies requested. I was only going to sell kegs to reduce consumables (bottles, caps, labels, boxes) and the time consuming process of putting all of those together. Just some of the things that helped me keep costs lower than normal, while selling my beer at the same price as other breweries. That's where I gained my workable margin.
A 10bbl system also wouldn't fit in my house and costs significantly more than a 1bbl sytem (at least initially as you mentioned). Although it might make more sense with a $250,000 budget, it's hard to jump into the brewing game that aggressively. The other garage brewery I mentioned also started with a 1bbl system and he's thriving today. So it is possible. Although he said he couldn't keep up with demand after just 3 months of brewing, he at least verified there was a demand for his beer before jumping to a larger more expensive system and subsequent taproom. It was smart of him to minimize his financial risk/burden at first to see if he could catch traction.
Toledo was very slow to catch onto the craft beer boom, so at the time there was definitely a demand. All of the local establishments I talked to were excited to hear there was going to be more local beer they could offer. Hell, a handful of breweries have opened since I moved 2 years ago. I had the timing right, but couldn't take the leap.
I do agree with your numbers and logic though for a more traditional startup brewery. Cheers!