From my experience the battery never was the reason for phone replacement. I always buy the best budget phone I can find and I keep it for 3-4 years. Usually the internal flash storage was the reason for phone upgrade. In some cases the internal storage was not enough for the latest apps that grow in size with every update, but in some cases the degradation of the performance of the internal flash lead to phone replacement. At the moment I have two phones lying around with user replaceable batteries that maybe lost 20-25% of the battery capacity but I’m not willing to replace them because the device itself is obsolete.
From friend and family experience who go for upper mid-range phones the experience is more the less the same. Phone replacement because of sluggishness and lost of performance not because of battery issues.
Where I live (south-east Europe) replacing internal battery or screen in a service shop is readily available and not very expensive.