By the way, first mentions of space expeditions in Mundus was not in some "MK's blabbery on forums". If you recall, Battlespire was an orbital battlestation. And space travelling was officially (oh by Lorkhan's balls!) canonized in Third Edition of Pocket Guide to the Empire.
If the space ships are similar in presentation to the Battlespire then I see no real worry, it's more an advancement of magic and a twisting of the fantasy genre.
Though I'm fairly certain the 9th era is a whole other beast, but hey, you probably knew that...
Anyway, if somebody likes space TES and someone else doesn't, who gives a [censored]? And I'm not hailing others opinions as something sacred to be treasured, other peoples opinions are generally worth about as much as my ass hair, especially on the internet. If they don't like space TES and you do, move on.
Anyway, my two ass hairs on the subject: 5th maybe, 9th never. I don't want my above average fantasy setting turned into a below average pseudo-sci-fi setting anyway.