[Too early. I heard in Russia they are discussing a ban on export of titanium to US and to sanction their companies from Boeings trade. That could halt 777 and 787 production for at least 11 months and would cost Boeing billions to replace Russian imports. On top of Max troubles probably it would be the last straw.]
Nobody says they are indispensable. But the idea is that not only Russia is the biggest sort of titanium. They also have the most expertise working on it. Not only the titanium parts in Boeing planes are made out of Russian titanium, they are designed and built in Russia as well. To think out how to get the raw material and to figure out building highly specialized parts to decent quality is a problem Boeing doesn’t need right now.