Free Public Blockchains?

> So my question is: wouldn’t it also be possible to make a blockchain system that supports smart contracts that’s 100% free to use

Free to use == free to spam.

Several blockchains with zero (or even just very low) fees have been targetted by large persistent spam attacks, blowing up their size by hundreds of Gigabytes, and resulting in calls for spam control measures. That led them to either raise fees, or require some form of PoW.

You may have a look at zeronet and ipfs, they're not blockchain but maybe interesting to you