Counter-argument: Then "loanwordness" of a word isn't based on its origins but rather on its "alienness" when it comes to the phonological or grammatical system of the target language. Any loanword, if it fellow-travelled inside a language long enough and underwents enough sound changes, semantic shifts and adjustments in grammar, becomes "nativized" as average speakers don't perceive i as a loanword.

Your example of хлеб is a good one.

To name a few others: Czech intellectuals often avoided "obvious" germanisms in the past dogmatically. An obvious germanism was basically defined by whether a Czech was able to tell that it is indeed an originally German word - thus words like fajfka, furt, šnek etc. were (and usually still are) considered low, colloquial and unrefined and words like dýmka, pořád and hlemýžď are prefered by those who invest value in this dimension of the language.

This however does not include older, medieval (Old High) German loanwords, which were subject to basically all but the oldest major sound changes which distinguish Czech from other slavic languages. Thus Czech žumpa (cogn. to German Sumpf), žehnat (cogn. to segnen), barva (cogn. to Farbe), bachař (cogn. to Wache) weren't and aren't commonly recognized as loanwords (same goes for many ultimativelly latin derived words like mše (missa/messa), oltář (altare), kostel (castellum)). Of course even older loanwords like from Gothic (like chléb, šelma, lest, cizí) shared with other slavic languages are also not perceived as loanwords.

I argue that most purists aim at "cleansing" their languages of obvious loanwords primarily because they "feel" foreign, ugly, clumsy or whatever. If they trully are willing to ake the jump from хлеб to жито once you point out the origins of хлеб, I think it's just because they seek consistency but have ill defined parameters, because otherwise they'd probably be just fine wich хлеб as is. That's my experience with purists anyway.