Even if we assume backdoors exist, your home is a low value target relative to the many large commercial operations, government offices, airports etc which use exclusively these brands. I think you're fine to leave them up, since they're still fit for the purpose you put them up for. Chances are you'll have no issues maintaining or replacing them in future if needed. At worst, hikvision et al could hypothetically see your surveillance footage, but they aren't gonna physically steal your stuff.