You aren't stuck with a demo. The "121 free" version includes:
The keyword "Eaglercraft Unblocked 121 Free" refers to a specific, highly optimized distribution of the Eaglercraft client. Here’s why this version has exploded in popularity: eaglercraft unblocked 121 free
Eaglercraft is a recreation of Minecraft's classic 1.5.2 or 1.8.8 gameplay that runs entirely in a web browser using JavaScript/WebGL.
Unlike the official Minecraft: Java Edition, Eaglercraft requires no installation, no launcher, and no official Minecraft account. It is not an official Mojang or Microsoft product, but rather a fan-made project. You aren't stuck with a demo