Distributional error probability of deterministic algorithm implies error probability of randomized algorithm?


Consider some problem $ P$ and let’s assume we sample the problem instance u.a.r. from some set $ I$ . Let $ p$ be a lower bound on the distributional error of a deterministic algorithm on $ I$ , i.e., every deterministic algorithm fails on at least a $ p$ -fraction of $ I$ .

Does this also imply that every randomized algorithm $ \mathcal{R}$ must fail with probability $ p$ if, again, we sample the inputs u.a.r. from $ I$ ?

My reasoning is as follows: Let $ R$ be the random variable representing the random bits used by the algorithm. \begin{align} \Pr[ \text{$ \mathcal{R}$ fails}] &= \sum_\rho \Pr[ \text{$ \mathcal{R}$ fails and $ R=\rho$ }] \ &= \sum_\rho \Pr[ \text{$ \mathcal{R}$ fails} \mid R=\rho] \Pr[ R=\rho ] \ &\ge p \sum_\rho \Pr[ R=\rho ] = p. \end{align} For the inequality, I used the fact that once we have fixed $ R = \rho$ , we effectively have a deterministic algorithm.

I can’t find the flaw in my reasoning, but I would be quite surprised if this implication is true indeed.