People moved away from steam when they realized you just needed a key to start up the engine of a diesel, as people wanted tighter timetables and faster transport, the days of taking it easy were fast becoming a thing of the past.
Not even close. The major contributing factor to steam's downfall was manual labor, pure & simple. Coming in close second, diesel's utility.
A train needing more than one steam engine required an extra engineer and fireman. Depending on operations, possibly an extra conductor, brakeman, & flagman. That's just running the thing. Plus you needed a hostler to keep the fire going and water in the boiler while it wasn't even being used. Steam required each road to decide where the power would be used and design accordingly. Look how few engines were ever duplicated from one road to another.
Diesel you just tuck another one into the train, lace up the MU hoses and plug in the jumper cable. Quick brake test and all control is with the engineer on the point. Diesels allowed the builders massive economies of scale: one model applied to all divisions of all roads. Its why we boiled down to two major manufacturers, GE & EMD.
When N&W tested diesel, they intended to only swap out switchers, a spot they could benefit from diesel's ability to be turned off in the summer. Road power was still expected to remain steam. They only began to fall from that position when they realized they couldn't buy the parts they needed to build their own steam engines anymore.
When NKP tested diesel, they compared a lashup of diesels against a Berkshire (ala 765) and the steam wiped the rails with the diesel train on a hotshot from Chicago to Buffalo.
New York Central tested an EMD E A-B-B-A lashup against an S2 4-8-4 Niagara. One steamer against 4-diesel locomotives each equipped with twin 2,000hp diesel engines. The diesel, after extensive testing, showed only a single digit of year-round availability advantage.
Chesapeake & Ohio's 2-6-6-6 H8 Allegheny generated 6,500hp at the drawbar in actual revenue non-test condition testing. They could haul 13,000ton coal trains singularly on the level. Keep in mind, this tonnage was in far more hoppers that we would run today due to increased car capacity. This meant the steamer had to overcome a far greater inertial resistance just to get started and a much greater rolling resistance as hundreds more wheels had to grind through curves and thump over diamonds, switches and bad joints.
And yet the steam could do 100mph... 844 is credited with 110mph. N&W611 is said to have a top speed of 90mph, despite tiny drivers for passenger service. C&0614 pulled a 20-car excursion train in 1998 at 80mph on Jersey Transit... At the ripe young age of 50years old! (Proof? do a youtube search for "C&O 614 80mph")
Taking it easy was never a phrase used with the steam locomotive. And in most cases... steam showed an advantage when it came to timetables. Even today, Amtrak's service isn't any better than steam did. (this of course exempts the NEC since PRR had electrified this in the early 1920's.)