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.
You need to pay to open channels though, which may not be ideal for some people. Off chain solutions certainly help, don't get me wrong, but I simply disagree with the notion that Bitcoin was built to help the poor. It
can, but Bitcoin is inherently anarcho-capitalist in that it promotes a free market resistant to outside interference, which is far from the socialist expectations of equality some people seem to have.