[strgcmc]

Plucking a number randomly from a real-world point-in-time snapshot of https://bitcoinfees.earn.com/, I see a suggestion of ~150 satoshis/byte, so for your example, transaction fees for an ideal/max size block would be 600 million satoshis = 6 BTC, which roughly is about the same as the block reward.

Needing to roughly double the status quo transaction fees to make up for lack of the block reward in order to provide equivalent security, doesn't seem ridiculous to me at all. And this is the worst case scenario (i.e. the reward will go from 6.25 to 0 slowly, but it can't go negative).