Here is a good article describing some of the limitations of sand we may face -
https://www.newyorker.com/magazine/2017/05/29/the-world-is-r...Humans brains replace themselves and the resources it takes to grow a new human (complete with a new brain) are much lower and easier to acquire than any artificial brain we may come up with. It takes awhile to get little humans up to speed, but they'll adapt to any environment you throw at them and in a time of strife will be able to make themselves useful way sooner than they do today.
Any time you start saying words like "indefinitely" you're falling into a trap. We may have "infinite" silicon available on the earth, but it's not equally accessible and it can't all be extracted and refined in the same way.