Galvanic noise isolation is not absolute. It offers a degree of noise attenuation and what it offers can vary depending on the quality of the magnetics used amongst other things. Data integrity isn't the issue in these instances. Data can exist uncorrupted in very noisy environments. But your average £25 five or eight port SOHO switch was never designed to have something as sensitive as a streaming DAC with a noise floor of -130dB in the analogue domain. This sort of device is hugely disrupting because they simply didn't exist when the standards were drawn up.As you imply a high quality Cat 5e cable (Connectix or Blue Jeans Cables) between switch and music renderer will galvanically isolate the hifi from the network. Switches regenerate traffic so a high quality one (e.g. HP, Cisco) will minimise noise. As long as the network is stable I don’t see or hear it as an issue.
I think, if the standards were re-drawn with things like the afore mentioned DAC or 4/8k UHD TV's it would look very different. But they won't be because for the very first time since it's inception we have WiFi fit for purpose. WiFi 6 fixes everything!