Oh this will be fun, the only gas harder to keep inside is hydrogen. While its absolutely true that you get a huge benefit in head flying by switching to helium it also means that there is pretty much a guaranteed lifetime cap on the usefulness of the drives. As someone who tries to run drives 24/7 for 3 years before tossing them this probably makes my life harder. Especially if the 'failure' mode is that they just get slower and slower as the heads have a harder and harder time staying on track as other gases migrate inside.