After reading all of this and me not claiming to be some expert. My two cents

Having grown-up in the "Schwinn business" and worked/mechanic in my family's bike shops in the early boom 1970s. ~ Even though at 56 years of age, I still live and breathe vintage bikes, and have a huge collection.

What for me are some points that newbies need to know and can often be overlooked.
1) People often buy a bike for the reason that they want to "get in shape" - And like exercise equipment, the thrill of that resolution wears-off soon after. Then the bike simply hangs in the garage rafters or on the wall for years, or even decades. So yard-sale, Craigs list deals are out there! (I amassed my huge vintage bike collection in exactly that manner)
2) Bikes built 20, 30, 40 years ago were made of much better materials than the big box store China stuff they sell now days.
3) High quality new bicycles cost a lot of money, and this is what drives the volume of sales of the China Wally World market. It's all about price.

Buy that nice older bike that was high quality back in its day, but has been hanging around the garage for 30 years. Even if you pay a price equal to the new China bike, and then pay the LBS to tune it and replace the tires, in the end, it is almost always a better deal. Even if it ends-up costing three times the price of the department store China bike, in the end, you have a better ride, much higher quality overall components, much higher reliability, and above all, something unique.

Regardless of collectable aspects, many of the older main-stream lesser expensive bikes were simply made better. Even a well-preserved, correctly tuned and set-up 1970s Schwinn Varsity will run circles around any Department store ten speed. And it will run and run and run and not break-down.