That's no longer true. I appreciate my headphone jack, but Bluetooth continues to iterate and 5.0+ has pretty good sound quality. If your specific hardware only supports 3.x, then it's true that it will never get better, but the standard has improved. Other improvements include better range and security, low power modes, and higher bandwidth.

Also mp3 is a file and compression format and can vary in quality by quite a bit, depending on bitrate. It's more useful to express quality in sample rate and bit depth, or bitrate. Bluetooth is capable of transmitting 16bit 44khz audio (CD quality) if both devices support the correct optional codecs.

That said, the wired connection supports whatever the DAC can handle as it's an analog signal, so theoretically even better than CD quality (though not really in practice since the source audio won't likely be better than CD quality, plus increasing the sample rate doesn't really improve anything since it would just add higher frequencies and CD quality is already at the limit of the human ear).