I’ve definitely replaced iphones because the battery wasn’t holding a charge. My understanding is that this is a pretty common issue, no?

That is your choice, however. Plenty of people choose to pay for a new battery rather than drop hundreds of dollars on a new phone. Apple still offers battery service for $69 for the iPhone 6, a phone released 12 years ago. You could have a new battery put in it today and then go a few more years. Nobody is forcing you to buy a whole new phone.