Then you're looking at it wrong. It's not a foundation. Foundations can't be completely optional.That isn't a problem, it's awesome! Why do you think it is a problem that Wild Sorcerers get a powerful ability as a foundation for their subclass? Looks like good design to me.
And if a DM thinks ToC is too powerful to have multiple times a day, then the player gets neither ToC (again at least) or dramatic rolls on the Wild Surge table.
Such tieing of a core concept to a powerful, yet optional, ability is the worst form of class design possible. You either have Tides of Chaos refreshing with every spell, which most DMs would find too powerful, DMs refreshing ToC on a limited basis, leading to them being unable to refresh it because players are holding onto it for a better time, or a DM that just flat out doesn't refresh it, which is just unfun.
Wild magic Surge should of just read "[...] the DM can have you roll on the Wild Magic table." and Tides of chaos refreshing should of been the feature with a 1 in 20 chance.