I hope you never experience this.
You're on a bike trip that took you months to coordinate with your friends. Maybe you bought matching jerseys. Maybe you haven't seen them in years and this was a time to catch up while on bikes taking in the beautiful scenery. The sun's warmth, the windy roads the trails. Everything is perfect....until it happens.
You're pedaling up that hill when you shift and you hear that noise. CRACK! It's followed by sounds of metal on metal and carnage. Maybe a few expletives. You just broke your derailleur hanger. There are other ways it happens. Maybe your bike falls over or maybe someone runs into you but at the very least, your derailleur hanger is doing what it's supposed to. It's supposed to break or bend before greater damage can be done to your bike. Regardless, it's not fun. How will you be able to continue your epic ride?
In the 90's when more bike companies started making their frames out of softer more malleable aluminum, they needed a way to reduce frame damage. The replaceable derailleur hanger was born. The idea was the hanger was cheaper and easier to repair than the entire frame. Before aluminum, frames were made of steel. My thoughts on steel....well....I don't wear skinny enough jeans or drink enough IPA to be an authority on steel.
Wheels Manufacturer is the largest manufacturer of after market derailleur hangers. They print this poster and distribute it to bike stores. It has over 250 different hangers for various brands and models of bikes. If you go to their website, they have over 500 derailleur hangers to choose from.
At any given time, how many of these do you think your local bike store carries?
At any given time, how much space and resources do you think your tour mechanic is going to devote to having a hanger for your 90s road bike you bought in college?
Probably not a lot. Probably zero.
Most shops just carry the current hangers and maybe a few left overs.
Now for the insurance part. If you have a big biking trip coming up. Buy a spare hanger and throw it in your gear with you. You might break chains, bend wheels or snap forks. Those challenges are usually easier to solve. A derailleur hanger is usually 2 or 3 days away from any bike shop. Just buy one and throw it in your and hopefully you'll never need to use it.
Your cost for this insurance is cheap. Much cheaper than canceling a weeks worth of riding with your people.
TLDR: When you're planning bike trips, buy derailleur hangers for your bikes just in case.