Pumping helium in & out of a zeppelin would be even more complicated than pumping water.

Or you could load a heavier material for ballast, and gradually transform it into a lead zeppelin.

But seriously, why can’t you just tie the zeppelin down? Is there not any sort of cable material with enough tensile strength?

Hmmm, running some rough calculations the passenger load probably wouldn't exceed about 50 tons for 500 people. Which is very doable to do with a tether. So maybe you're right! Solar-powered passenger airships could be the future. It's cargo airships I'm thinking of that really have these drawbacks.

And for a cargo zeppelin, swapping out full shipping containers for weighted ballast shipping containers seems pretty easy. Though I seriously doubt cargo zeppelins make any sense at all.