@neologist,
Of course we all tend to operate as naive realists in daily transactions. The
raison d'etre of philosophy is to question the basis of such "normality" especially in
social transactions where matters of differential existential belief tend to come to the fore. But a secondary consideration of more recent years has involved the status of the "observer" in frontier physics. For example physicists are reluctant to state what "an electron IS", or even what "a particle IS". At the very least, therefore, the nature of
is-ness, cannot be assumed by philosophers (as you have already pointed out) and any subsequent use of "is" at
this level of analysis cannot signify epistemological or ontological closure. Indeed, that is why pragmatists have abandoned such issues as intellectually futile.