As I posted above, batteries typically last 1,000 to 2,000 cycles, which gives most EVs between 200,000 and 600,000 miles of battery life -- and at the end (barring going out with some sort of fault) you can still manage 70-80% of the original charge.

We don't really know for current EVs how calendar aging will affect them, but we can look at older EVs with now-obsolete batteries. Some of them are fine, some of them aren't. Chevy famously did a recall on their Bolt battery packs, and a lot of Nissan Leaves had their batteries go bad. The problem with the Nissan Leaf is they only had passive cooling on the battery. So, lesson learned. Basically everyone does active cooling now.

In theory, lithium iron phosphate batteries should last a very long time. That's what I'd go for if you want the best longevity, but they also have a lower energy density and can be damaged if you charge them in below-freezing temperatures. (Presumably all the cars with LFP have smart charging systems that disable regen and refuse to charge when the battery is too cold, and hopefully most of them have battery heaters.)

With batteries there's this weird dynamic that if it last for a reasonably long time, by the time you need a replacement the new batteries /should/ be much cheaper and more energy dense. Unfortunately, there aren't really any companies making 3rd party aftermarket batteries for old cars. Maybe that will change, and maybe the industry will standardize on some generic battery module design. For now, the replacement battery options seem to be a) buy a replacement from the OEM, b) find a replacement from a low-mileage junkyard vehicle or c) reverse engineer the BMS and install an equivalent custom battery constructed from new or used cells/modules, possibly using some kind of custom-fabricated battery box. It'd be great if you could just walk in to any auto parts store and buy a replacement battery pack, but we're not there yet.