It's not sustainable. It's clearly nice for now, but they aren't going to continue to give out games like this forever. They are just trying to get people to introduce themselves to their economy and then once the numbers are there, this will all dry up.
Obviously, people don't give stuff away for no reason, however the "string attached this time is "enticement".
Same here, I wonder how many people act the same, I think only one or two of my friends actually purchased something during the last big EGS sale.
There are probably a ton, but this is like the news feed and messages on Facebook - most people go there for it and only it - but some will end up buying into the ecosystem, clicking on a publicity, etc. Epic seem to be aiming into building a critical mass of gamers who go there to play, therefore they will look at their store when it's time to make a purchase.
It took years for me to even consider buying games on Steam - and I only ever bought them through humble bundles and really cheap on sale games (like I look at the sub 5$ / 10$ offerings).