I think it is possible to build a better system from scratch, and there are likely space aliens that have such better systems.

But ultimately a computing system should do what its users want, and the market has favored backwards-compatibility for a long time. If anything, it's a little alarming how old software and hardware can quickly become useless or obsolete. For example, any web browser that doesn't support any encryption better than, say, SSLv3, is cut off from a significant fraction of the Internet.