Can Bitcoin remain tamper-proof in the long run? We use block-level data from the Bitcoin blockchain to estimate the impact of congestion and the US dollar price on fee rates. The introduction and adoption of the segregated witness (SegWit) protocol allows us to identify an aggregate demand curve for Bitcoin transactions. We find that SegWit has reduced fee revenue by about 70%. Fee revenue could be maximized at a block size of about 0.6 megabytes when SegWit adoption remains at current levels. At this block size, maximum fee revenue would be equivalent to one-eighth of the current average block reward. Hence, large sustained price increases are required to keep mining rewards constant in the long run.