Lets take the question as being "suppose the hieroglyphs had never been deciphered"
I think this is unrealistic. People will find a bilingual at some point to either crack them, or crack cuneiform and them turn unto cuneiform-hieroglyphs (IIRC, cuneiform was partly deciphered through hieroglyphs IOTL).
Even without this, at some point computation and mathematical theories behind encryption theory will develop enough to make pull a Ventris on them, working backwards from Coptic. It would be far more difficult than what Ventirs actually did on Linear B, of course. So, I think that it is likely that,
by this day, Ancient Egyptian would be readable for scholars. However, this would set back Egyptology more than a century and half. Considering what a mine of knowledge on Antiquity Egypt turned out to be, the consequences would still be very significant: for one, Egyptian nationalism would be a very different beast.