Science

H atoms and H2 molecules are sufficiently low in mass to escape from Earth's gravitational field at the temperatures available. H2O and O2 are heavier and escape much more slowly. So escaping H and H2 allowed some of Earth's early water to vanish.

As long as the ozone is in place (and we banned CFCs to ensure that is) the rate of photodissociation is very unlikely to ever regain the importance it once had, since the ozone absorbs UV very effectively -- that's why it's called the ozone "shield", and why severe sunburn and skin cancer were expected to increase if we didn't ban CFCs. Of course most plants and animals would have been affected as well, but even that impact if due to less energetic UV than is required to photodissociate H2O. In the very outer reaches of Earth's atmosphere, some photodissociation still takes place, but it involves very small amounts of material. Really short UV never makes it to the surface.