The thing you probably need to know about cycling is that everyone crossing the line in the same group gets the same time regardless of if they were first or 10

The thing you probably need to know about cycling is that everyone crossing the line in the same group gets the same time regardless of if they were first or 100th. The difference in energy required to ride on the front of a small break away vs in the middle of a peloton can be as high as 50% on a flat stage. Given that, realistically what would happen if you attack on the last flat day is that the peloton would just follow you and wait for you to tire yourself out. The domestiques on the front of the peloton can we swapped over time to give them a rest so that the peloton never slows and in fact increase its speed over time. This is why the sprinter's teams almost always catch the breakaways and why the beakaways are allowed to go up the road in the first place. Sometimes the peloton gets the sums wrong or doesn't factor in the affects of the wind and the break succeeds but not very often. By the last day of the Tour the leader can have minutes on second place so the chances of them making that up in a break are pretty small. Aerodynamic inefficiencies and tradeoffs are one of the things that make cycling such an interesting spectator sport for TV.