Probably not even that. Venus will have lost most of it's hydrogen as it stopped having liquid water. Which is a chicken and egg problem sort of, because the presence of life protects against that, by providing a sink for CO2 and by generating an ozone layer. the former keeps the water liquid (allowing oceans to store more CO2) while the latter protect water in the atmosphere from photo-dissociation

A magnetic field also may not help that much either. Hydrogen is light enough that the Earth losses it anyways. As long as the hydrogen remains as water, its not going to be lost that much faster. Meanwhile a run away green house effect would still end with Earth striped of its hydrogen over time. It'd just take a bit longer.