  Recently, it has been found that complete resolution of the Hubble tension
might point to a scale-invariant Harrison-Zeldovich spectrum of primordial
scalar perturbation, i.e. $n_s=1$ for $H_0\sim 73$km/s/Mpc. We show that for
well-known slow-roll models, if inflation ends by a waterfall instability with
respect to another field in the field space while inflaton is still at a deep
slow-roll region, $n_s$ can be lifted to $n_s= 1$. A surprise of our result is
that with pre-recombination early dark energy, chaotic $\phi^2$ inflation,
ruled out by Planck+BICEP/Keck in standard $\Lambda$CDM, can be revived, which
is now well within testable region of upcoming cosmic microwave background
B-mode experiments.
