Mike 🇬🇧 🇪🇺 on Nostr: I'd generally agree. As far as I could see it looks as though this part of the canal ...

I'd generally agree.
As far as I could see it looks as though this part of the canal is on an embankment.
•Major Edit to the below speculation •
The canal forum - link below - presents good, if not conclusive, evidence that a culvert running below the canal had collapsed causing the canal to collapse into the culvert
The culvert appears to have carried a stream below the canal.

https://www.canalworld.net/forums/index.php?/topic/126111-major-breach-at-whitchurch-on-llangollen/

•Original Speculation •
Years of mechanical abrasion of the canal bottom (from boat movements) perhaps alongside a different approach to maintaining the canal - including dredging and towpath maintenance - as a minor factor, may well have a role to play

•Further Comment remains relevant •
Whatever the cause it's going to be a huge & costly job to reinstate the canal for a charity that does not have the funds for such disasters.
Let alone the loss of the boats, which may be homes as well as investments, and the disruption to multiple lives.