Yes, perfect filters don't exist, that's one issue. The other issue is that using Opus now forces you to use that one filter the encoder chose to use. As weird

Yes, perfect filters don't exist, that's one issue. The other issue is that using Opus now forces you to use that one filter the encoder chose to use. As weird as it may seem, there are various reasons for preferring a lower-quality filters if your source input signal has a low sample rate to begin with (the result may sound fresher due to non-perfect filtering, which can be preferable in some situations). With Opus, the decoder would first have to downsample the 48kHz to the original sample rate, only to upsample them again with a lower-quality filter to achieve the same thing. I think avoiding all those extra steps has its advantage.