The high transaction fees recently are pushing out a lot of the traditional use cases for Bitcoin. Bitcoin Cash appears to be in a position to inherit those use cases.

That's unlikely. Bitcoin cash isn't accepted anywhere yet, and unlike Bitcoin it does not have Segwit, and thus no layer2/lightning network support. It's 8MB blocks only up the on-chain scaling from like 3 tx/sec to ~20tx/sec. That's a negligible difference when compared to credit cards, paypal, etc. Bitcoin will only ever reach that kind of capacity with 2nd layer lightning network, or something like it.

When will btc adopt lightning?

Segwit was just activated on Bitcoin around two weeks ago. Among other things, segwit includes a fix for txn malleability needed to facilitate a lightning network implementation on bitcoin. The lightning network alpha release has been implemented on Litecoin, and has been on the Bitcoin testnet since the beginning of the year. My guess is it will go live before the end of the year, and adoption will ramp up next year.

Well, don't these charts only reflect new people buying Bitcoin (as an investment)?

https://blockchain.info/pt/charts/n-unique-addresses?timespa...

You could probably look at the distribution of transaction sizes to try and determine how much is investment vs commerce. A bunch of $1-10 transactions are less likely to be speculators getting into a position.

Those data don't differentiate bitcoins bought for investment and for trading.

It does, because investors typically do not transact in Bitcoin. Buying and leaving BTC in an exchange wallet does not create a transaction on the blockchain.