What are the reasons why a iPad Pro 2 is no longer able to hold a charge

I have a iPad Pro 2 that is no longer able to hold a charge for long and was wondering if all it needs is a replacement battery or whether something else could possibly be the culprit internally?

Only asking because although the iPad Pro 3 is out, the Pro 2 in my opinion isn’t that old. Also, I hope that the problem isn’t going to persist once I do replace the battery, as out of warranty repairs are quite expensive.

thank you