1
In our previous article, we suggested (based on significant evidence) that those who tried to reinstate daily ברכת כהנים were stopped from שמים because they were מוציא לעז על הראשונים in the process. This means that by means of instituting ברכת כהנים on a daily basis, one thereby implies that their predecessors were willfully neglectful of their halachic obligation to do so themselves.
We will offer a more precise definition of הוצאת לעת על הראשונים in a future article אי"ה. What remains to be explained here, however, is why, indeed, is being מוציא לעז על הראשונים so severe as to warrant our continued neglect of (what would otherwise be) absolute halachic obligations. -?-
2
In the entry for חשד (suspicion), פלא יועץ exhorts its readers to, ideally, not eat anywhere outside their own homes, for fear of ending up eating something not kosher, or something that was warmed improperly on שבת or the like.
פלא יועץ warns, however, that not everyone is ראוי לכך to take on such a חומרא, as doing so would entail, among other problems, being מוציא לעז על הראשונים, in that taking on such a practice would seem to be insulting the previous generations who, by implication, were ח"ו not so punctilious about שמירת כשרות.
Now, one must keep in mind that פלא יועץ here is discussing a חומרא, meaning that, strictly speaking, there is a principle of עד אחד נאמן באיסורים, namely that one witness (even oneself) is qualified to testify with regards to matters of איסור והתר such as כשרות. This is the halachic mechanism which allows us to eat each other's food. To this end, we derive from here that, so long as there is what to rely on, it is better to keep to the strict הלכה without הידורים if said הידורים were to imply that the previous generations were neglectful of halachic strictures.
3
With regards to daily ברכת כהנים, there are many פוסקים to rely on to the effect that ברכת כהנים does and/or should not be performed daily, to which end it is better to continue relying on them (even where their halachic reasoning contains many logical and/or halachic difficulties) rather than to imply that the previous generations were neglectful of their religious obligations.
Similarly, there are opinions to rely on with regards to ruining a kosher טלית before burial, saying פיוטים where interrupting the תפלה would otherwise be problematic, and certainly for not inserting an extra פיוט into the liturgy against the established local custom [1]. To that end, it is better to continue relying on those opinions, even if their halachic reasoning is problematic, rather than to imply that the previous generations were neglecting what they were supposed to do.
Appendix
פלא יועץ does not, in fact, discourage people from not eating each other's food, rather only cautioning that doing so could potentially be מוציא לעז על הראשונים in the process. In fact, this entire הנהגה is referred to as a מדת חסידות that is not for everyone. The clear implication of that is that one for whom מדת חסידות, as a function of their overall religious piety, is appropriate, then not eating anyone else's food out of concern for lack of כשרות would then not be out of place.
With this we can explain how ר נתן Adler, who performed ברכת כהנים daily in his private בית מדרש, was not quite punished from Heaven but rather down here on Earth, in that he was placed into a חרם by the Frankfurt Kehilloh. This would perhaps be a result of the fact that he did not specifically intend to institute daily ברכת כהנים in the entire world but rather just in his own רשות היחיד. In fact, there could easily have been dozens of כהנים in Frankfurt at the time, yet we have no specific record (to my knowledge) of them being invited to his בית מדרש for the opportunity to perform ברכת כהנים on a regular day.
[1] for details see previous article and references there