You are right. The only time when it makes economic sense to solo-stake is when the following is true:
YourStake(%)-PowerCosts(%) > (1/k) - StakingPoolFees (%)
k is the approximate number of StakePools Cardano will have. If this number is 100 then you’ll probably have to have >=1% of the total stake to make solo staking worth your while.
Depending on how much ADA will be available for staking (I assume a lot, in the beginning) it is more probable and economically attractive to stick with a pool. As the utility value of ADA grows, we will see more people use it for fueling their smart contracts on Cardano’s network. That will create an upward pressure for prices and provide incentives for people to sell their ADA on exchanges. As they sell their ADA on exchanges, i.e. as it gets used as a transactional unit on Cardano’s network, the total stake will shrink and make rewards more attractive for the rest of those who stake.
It is a beautiful setup and I can’t wait for it to go live. An absolutely amazing project in crypto with some serious applications! It will easily eclipse the now-dying Ethereum as it will have a far superior platform developed by real scientists!
EDIT: cleaned up the logic and typos.