Middle class generally means people whose incomes are in the middle half (ranging from 40th to 60th percentile to the 20th to 80th).
If you want to pull out your own new definition based on whether their income comes from work or from return on investments, then I’d still point out there’s a large number of people who do both, especially when compared across the entire life cycle including retirement. So if you insist on this alternative definition, you still have to account for the big chunk of the population who do both.
And just like that you’ve shown that gravity batteries aren’t feasible.
Storage is going to be a big part of the solution going forward. But it’s going to be chemical batteries and thermal batteries, not gravity batteries.