The problem with the bike shaped Walmart thing is that it is absolutely irreparable.

Got a $200 bike shaped thing first thing when I got a job (technically Kmart but otherwise the same). Dumb mistake. The crank gear bent out of shape before too long, just from applying pedal power to climb a hill. On another bike, that’d be $20 in parts and $40 in labor maybe? On this one it was basically impossible as it was not designed to be replaceable or even serviceable and I had to throw the bike away.

Never mind that they're assembled by Walmart employees who have no idea what they're doing, and can be quite unsafe. You wouldn't buy a $10 airbag replacement for a car, and you shouldn't buy the cheapest bike possible. This is safety equipment; a failure at speed can be quite injurious to your body.

Exactly. Cheap bikes are made out of aluminum, which can fail at any time without warning and cause injury or death. Much better just to pay extra for carbon / steel / titanium. Carbon still fails, but there are more likely to be warnings first, and even if it does fail completely you're not likely to puncture a lung or whatever from it getting impaled through your chest. Steel and titanium shouldn't fail at all unless you're in a bad accident first. Bikes are great, but you need to take seriously the fact that they can cause serious injury or death.

Where are you getting this from? Carbon fiber is more failure-prone than aluminum is. There are some very nice bikes made out of aluminum (up to the several thousand dollar range). Steel vs aluminum is more of a choice in ride comfort vs weight.

Aluminium is fatigued with use over time, so will at some point fail, even if it's not subjected to major trauma. (This is opposed to steel which is only fatigued by major trauma, and carbon fibre which cracks when you look at it wrong.)

This seems like an overblown concern. How often are aluminum frames actually failing? How many miles and how many years do you have to put on one before it fails? And how do these figures contrast with steel? And don't forget that steel is more susceptible to rust than aluminum is, so depending on climate and salting conditions the steel might fail first.

My aluminum road bike cost me $600 new and I have four years of daily commuting on it so far. It's still in good condition and I expect to get many more years of riding out of it. At this rate I'm gonna have it for longer than the average car ownership period in the US (which is about 7 years).

The fears are being blown out of proportion.

It is definitely an overblown concern. I just wanted to present the steel man version of the grandparent's argument.