Draining lithium reserves isn't that important - batteries don't use up the lithium, once the battery dies you can just suck out all the lithium and re-use it (and battery electrolytes are ~100% lithium, compared to lithium ore/brine being anywhere from 0.1% to 15% lithium - an order of magnitude difference). And since modern batteries are more efficient than old batteries with the same amount of lithium, we effectively increase the circulating lithium capacity over time.

In 75 years we won't need to extract more lithium - except the fraction needed to replace permanently-lost batteries.

Incidentally, you should be very careful when talking about "<resource> reserves", because the definition of a reserve is usually "<resource> that is profitable to extract" - and when we "run out", prices will go up and thus currently-unprofitable sources will become profitable, and POOF! Our <resource> reserves have increased, purely through the power of semantics.

Also, over the decades resource extraction becomes cheaper and thus more sources become profitable.

Personally though, I don't think any of that will matter -IMO the future is proton batteries, AKA Hydrogen batteries (which use an electrolyte of "ionic hydrogen", H+, which has 1 proton and 0 electrons - people claim lithium is the lightest metal, but it has 3x the protons of hydrogen). I think that the recent TABQ batteries, or something like it, will become commercially viable within 75 years (although who even knows what batteries will look like in the year 2101).