Other than your carrier give it for free or cheap, I don’t really see the reason why should you buy new phone. I’ve been using Redmi Note 9 for past 3 years and recently got my had on Poco F5. I don’t see the point of my ‘upgrade’. I sold it and come back to my Note 9. Gaming? Most of them are p2w or microtransaction garbage or just gimped version of its PC/Console counterpart. I mean, $400 still get you PS4, TV and Switch if you don’t mind buying used. At least here where I live. Storage? Dude, newer phone wont even let you have SD Card. Features? Well, all I see is newer phones take more features than it adds. Headphone jack, more ads, and repairability are to name a few. Battery? Just replace them. However, my Note 9 still get through day with one 80% charge in the dawn. Which takes 1 hour.
I am genuinely curious why newer phone always selling like hot cakes. Since there’s virtually no difference between 4gb of RAM and 12gb of RAM, or 12mp camera and 100mp camera on phone.
Consider buying a phone which lets you change the battery considerably easy. I watch teardown videos of phones before i buy one to compare the process and the likeliness of me breaking something in the process. Of course not everyone is going to do this, but you could ask a friend to do it (i changed batteries for phones of at least 3 or 4 people by now).
I’d say that’s only half the problem. While ease of disassembly is a factor I’d personally consider when buying a phone, I feel like the more difficult part is finding a good quality battery replacement. For the most popular phones (Galaxy S series, iPhones, and a few others) you can probably find a battery at a reputable site like iFixit, otherwise you’re stuck with ordering something that supposedly matches the part number on Amazon or some sketchy Chinese site. Is it a new part or a refurbished OEM battery? Is it anywhere close to advertised capacity? Will it work any better than the used battery you’re replacing?
My tip would be to look for replacements which are advertised to work with the specific phone model but are not necessarily original replacement parts, bc like you said they tend to be fake and of bad quality. There are a number of brands who have specialized in this market by now. There are also very few brands who sell original parts via ifixit (e.g. google for pixel phones)
Also, if you want to replace a battery stay away from samsung. They glue the batteries in so tight that you have to use alcohol to loosen up the adhesive.
Also also, if you want to replace anything, stay away from apple. They have a history of bricking devices software side if they detect third party parts.