Earlier quoted context omitted.
If the bike breaks mid stage, you either switch or you are DNF. Except when you allow riders who did not finish one stage to continue on the next, which would be totally ridiculous in an endurance event. In that sense, The Tour very much _is_ a single race (for the rider, not for the bike(s)).
Okay, so then allow for a bike switch if the bike breaks, as judged by a race official. One bike per stage doesn't seem like it would put too much strain on the hardware compared to one bike per year. Surely it would be possible to build bikes at a high enough quality that the number of DNFs would be so incredibly low as to be negligible. I love these Internet arguments that are like "okay, we have to argue and nitpi…
It's a race. The entire point of switching bikes is that swapping bikes has less impact on the athletic comparison than a lengthy repair and waiting for an official would defeat that. Besides, if you have a race official at hand, you might just give the old bike into his custody for engine detection, defeating the purpose of the maybe-no-swap rule.
The entire point of not swapping bikes would be that it could make engine cheating detection slightly easier. But that is far from necessary, developing reliable procedures for traceable bike swaps is far from impossible.
The entire NASCAR comparison is off anyways. Performance bikes don't really suffer much linear attrition over the course of a race. Even over three weeks of TdF, the distance ridden there is just within the typical service life of the most short-lived expendable part, which happens to be the chain and not the tires. Breakage within a race is therefore entirely stochastical, mostly getting a piece of gravel in the wrong spot, both for chain and for tires. Motor racing, on the other hand seems to be full of trade-offs between deterioration and performance, just look at tire consumption. And where motor racing is dominated by massive technology budget differences between teams, in cycling all participants in a single race have pretty much the same level of support: ranging from none at all on the lower levels of amateur racing to the full set of neutral support vehicles plus two cars per team at the high end.
> What's so bad about DNFs for equipment failures?
Stage races. DNF one stage means DNF for the whole set of stages. That's the entire point of an endurance sport. I suppose that in NASCAR, drivers who DNF for technical reasons in the first race don't have to wait for the next season to be allowed to start again? It's just no valid comparison.