I'm pretty sure that both Android and iOS stores continue to lack a good "paid upgrades" mechanism and it's been a developer complaint for well over a decade. I've used a few apps that came out with upgrades/rewrites and offered either a free upgrade (if the old version was still installed) or possibly a temporary discount, but there are just as many that simply get abandoned because it's not feasible to get a constant stream of new purchasers to remain viable.

Notably I'll throw in a few things like PocketCasts, Nova Launcher, Gentle Alarm by Mobitobi, a variety of camera and gallery apps. Some of those shifted to subscriptions (PocketCasts after being sold twice), others just got shuttered.