Bitcoin's cost effectiveness was always a secondary consideration. It was just one of those things which happened to work out with Bitcoin's decentralization at the time. Things have changed since, and it's not even always true nowadays anymore with fees surging every now and then. The poor can't afford to wait for fees to go down to spend.
The main chain fees are definitely not stimulating use in third world countries, but that's what LN is for. Technically speaking you only need to download and install an app and you can instantly receive payments.
It's already possible on a smaller scale right now, but with time it will grow large enough to please the group of people being open to it. I doubt all people will use LN, which just isn't realistic.
Just like there are people who use PayPal, there are people who use other financial institutions that offer a similar service. Competition in the payment industry is extremely tough, and Bitcoin will struggle too.
It's going to be interesting to see how Libra will be adopted when it finally launches. Many people in third world countries do have a Facebook account, so the step to Libra isn't all that hard.
Libra will also enable payments between family members which brokers currently charge 10% of that amount as fee, so we'll see where it goes.