Hamilton may have felt the tyres were fine with about 20 laps to go. But what would have happened if it was just 5 laps, or 2? Remember, what happened in Canada 2012. Alonso looked fine most of the race, but was lapping 4-5 secs off the pace in the final laps and dropped from 2nd to 5th in no time. Also remember, what happened to Vettel in Belgium, when the risk was taken and he suffered a puncture.

I don't know, what were Mercedes' calculation. Probably they feared a complete drop-off near the end of the race. Because they were running comfortable 1st and 2nd, and had no reason to fear competition from behind.

Tyres feeling "fine" with 20 laps to go means nothing. Probably Merc feared a complete drop off or a puncture in the last couple of laps, which could deny them the comfortable 1-2. If this fear was justified, I don't know. We don't have proof, just guessing.