We;re not talking transonic gliders here, and 30,000 ft is not abnormal operations.
If you want a more realistic example, take a glider with a Vne of 120kt operating at 15,000 ft (commonplace in the Western US, Australia, New Zealand, the Alps, South Africa, and in Scotland and Wales in the UK).
An online calculator tells me that an IAS of 120kt translates into a TAS of 156kt. That;s way beyond what an airframe (brand new) has been tested to (135kt) and is probably into flutter and structural damage territory.