Still far away until max supply reached but my opinion if the overall market capitalization of cryptocurrency reaching significantly in the future Ethereum value will automatically go up but only if there's no competitor that somehow could make ethereum kinda obsolete.
Bitcoin supply will last more then 100 years. But yes there will b very little left 20 years form now. I dont think 20 years is soon. Bitcoin exist 10 years. This is not an immediate problem Bitcoin have but is a long term problem that will hopefully be solved.
True since block reward becomes more fewer and fewer but it's true that bitcoin max supply will be reached far in the future.