[snissn]

There's a question that you're missing or ignoring about how long the 51% attack is sustained. Sustaining a 51% attack for one day is very difficult, doing it for one week or one month is proportionally that much more expensive.

[cdetrio]

Why is mining an attack chain expensive? If you mine on the honest chain, you earn the block reward for each block you mine. If you mine an attack chain with 49% and your attack fails, then you sacrifice those rewards. Suppose you mine say 10+ blocks with 51%+, over the next three hours while the rest of the network only finds 8 blocks. Then your 10 blocks become the longest chain and you earn the block reward for all 10 of them.