simply switching sources could case a change in the way the system sounds. If this change were negative when switching from a digital to analog source then some people might place the blame completely on the source media rather than realizing that a similar negative effect could also happen by switching to a different digital source.
Precisely. I have built and tweaked my system for over five years to sound good with my one and only source, a CD player. Now that I've achieved the sound I prefer, adding a TT to my system ... it may not sound as good as it could were I instead to have built my system using only the TT as a source. Had I done that, after five years of swapping gear and tweaking, striving for the perfect sound (to my eras), I might have ended up with a completely different system. One where adding a CDP as a source might not sound as good as it could.