“The people dismantled the sheikah tech because they didn’t want another calamity to hijack it”

This is exactly my personal headcanon, regardless of what Nintendo says. And TBH they contradict their own game with that statement, because there IS still some of the old tech left. For example, the new Skyview Towers: The terminals look almost identical to the ones in the old Sheikah Towers, and the platform that launches Link into the sky looks like a repurposed shrine travel gate. Link gets grabbed by mechanical arms/tentacles that look completely identical to Guardian legs, and they attach a cable to his belly that glows in blue and orange in the same manner (and color shade) as the glowy parts of non-posessed Guardians did (the possessed ones had a purple/pink glow, but the "hostile" shrine guardians that were controlled by the monks had an orange glow, and the non-hostile Sheikah Tech was glowing blue). And the "Purah Pad" is nothing but an upgraded and rebranded Sheikah Slate - where did Purah get that tech from? And Robbie still has his little robot "Cherry" sitting in the Hateno Tech Lab, obviously functional. AND you can still find the glowing mini daggers they formerly used as arrowheads for Ancient Arrows, which still work the same way as before (total annihilation of non-boss enemies)

That's a lot of random coincidences for allegedly "completely disappeared" technology.

...and if they wanted the leftover Sheikah Tech to canonically have been vanished completely, they would have needed to only mention it in some in-game dialogue that the same Gloom clouds that shot out of the chasms and corrupted/decayed random weapons also deliberately attacked any Sheikah Tech that was left in the open as well, disintegrating it. Parts that had been locked away by then (like inside the Tech Labs or in locked treasure chests for example) wouldn't have been affected if the Gloom couldn't reach them, explaining why SOME stuff is still present.

just saying "it was magic and went poof for no particular reason" sounds lame in comparison IMHO.

EDIT/PS: also, in one of the after-credits scenes in BotW, Zelda mentions that Vah Ruta has stopped working and she wants to investigate to find out why. We still see the Divine Beasts in the background of those scenes, and IIRC that cutscene happened a few days or even weeks after Calamity Ganon has been defeated. If the tech vanished "after Calamity Ganon was sealed away" why is it still present in those scenes? It would be weird for that stuff to keep sitting around in the overwold for an extra month or so before randomly disappearing.

As to why it only happened every 10000 years (if it even happened more than twice?) I’ve got no idea.

If I had to hazard a guess, the "ten thousand years" aren't exactly 10k years in the literal sense, but instead just mean "a very very long time ago".

I mean, if someone today says stuff like "there is a myriad of stars in the night sky" then they also do not mean that there are exactly 10k stars (because that's the literal translation of "myriad") but instead mean that there are too many stars to count. Maybe the same applies to BotW?

EDIT: I just checked wikipedia out of curiosity and it seems that "ten thousand" really does mean "numberless or infinite" in Japanese. I wouldn't be surprised if Treehouse of America accidentally went with the literal translation when a metaphorical one (like saying "an eternity ago" or something similar) would have made more sense for Westeners.