Misbatzos Zahav (9) gives two possible reasons:1)whatever the Gemara and poskim brings down as rui l’hischabed is forever even if one doesn’t think so,and l’chumrah whatever people call rui l’hischabed it is not batel even though it is not mentioned in poskim.2) Whatever the Gemara and poskim say is held until it is clear that it is not anymore,and everything else is not rui l’hischabed until it is clear that it is.The Misbatzos Zahav says in a case of issurei D’Rabbanan one should follow the second one.
