I think it's an incorrect assumption that you'll be coming back as a junior (on their part, not necessarily yours). Senior Devs learning new skills do not get bumped down a pay grade or more, with the rare exception that their old skills are totally worthless.

I think the idea that 20 years of experience in mainframe hardware warrants a senior or even mid level web development jobs is just as misguided as the idea that a college degree warrants any job just by virtue of having it.