I have managed in the past some high production sites (road tunnels) with a specific sequence of operations and having a jumbo or a wheel loader or an excavator down meant severe delays not only in excavation, but on all the following works (concrete pouring, impermeabilization) and on all the "ancillary" works (ventilation, electric cabins moving, etc.).

One of the machine at the excavation front broken for more than a few hours would have meant literally days of delays, complex scheduling modifications, etc., a total mess.

Correct, but the tradeoff calculation includes offsetting the expensive repair costs with potential increased productivity. Or likewise with the other option, the "savings" from cheap low-tech tractors may be negated by lower productivity.