Midgarsormr had been keeping a vigil over the lake from even before the time of the Allagans, so the Crystal Tower cannot be the reason why he was there. It's implied that him being there was part of the pact he formed with Hydaelyn upon first arriving on the star..

Considering that they suddenly made the horn from Seal Rock relevant again in the Alexander storyline despite the thing only getting a single passing mention from Nabriales prior, I don't think it's safe to assume that Silvertear is irrelevant at this point, particularly when it's specifically been hinted as being of importance in some of the lore panels.

It might not be part of the MSQ necessarily but I'd be really surprised if it wasn't at least involved in one of the post-MSQ trial/raid storylines.