Why are you so sure that Grin can be a rival to Bitcoin? even the privacy coins like Monero have not been able to compete with Bitcoin, even though there are many of the best coins and more than Bitcoin, but why everyone tends and prefers Bitcoin.
Bitcoin will always be king. Another word for rivalry is a healthy and competitive relationship. Just like Monero or any other top tier coin no one will ever come close to Bitcoin but a healthy competition is helpful to all the crypto community.