I have decided that it is going to be pointless to use the Tarrasque to threaten the PCs.
Instead, if I ever decide to use the Tarrasque, the PCs' job will not be to destroy it before it kills them, but to destroy it before it levels a town or city and eats the inhabitants.
Death is not the only way to fail.
This is great advice.
The Tarrasque is more like a natural disaster than something that needs to be fought. Like Godzilla the Tarrasque is a symbol of destruction and apocalyptic force.