Mike --
Do you own any computers, tablets, or "smart" phones. The one thing that's obvious about these devices is that if you can buy one on the open market, it's already out of date. Change comes daily and sometimes hourly in the electronics field. Things were much the same back in the latter 19th and early 20th century with metallurgy, shipbuilding and power plants. Ships not yet completed were outdated by the laying down of a more advanced design. Dreadnought was a revolution because if was the first capital ship (major naval vessel) to come all-turbine from the builders.
Even so, as you pointed out turbines weren't new. The first vessel to prove these machines could work was Turbinia built in 1894 by Ccharles A. Parson who invented the modern turbine a decade earlier. He commissioned Turbinia to prove his concept and that it did. At the Spithead Navy Review of 1897 it turned up 30 knots or so and cut didoes around the Royal Navy. Things weren't exactly "tickety-boo" with Turbinia. The problem of gearing down the high revs of the turbine to the slower revolutions needed by a propeller in water plagued development of the ship which, though fast was only about 12% efficient.
Looking at the flux in shipbuilding of that period, it's obvious H&W with White Star's consultation decided upon a rather conventional design of proven reliability for the Olympics. Instead of being the most modern, they were to be the most reliable. Or, their prime movers were to be so. And, the career of "Old Reliable" RMS Olympic proved that point valid. The low pressure turbine was a clever idea to get one foot into the tent of modernity without risking too much on unproven equipment. If you really study the Olympics, though, they are twin screw vessels at heart, augmented with an auxiliary third screw at speed.
One thing to understand is that the cost of a breakdown of an engine in a passenger ship is far more than just parts and labor to repair it. You have to add in the lost ticket revenue for trips missed while the vessel is incapacitated. And, there is the loss of "old" customers who are forced to try a different shipping line, find it to their liking, and never come back. There is also the cost in terms of lost crew. Key ratings could not sit ashore in those days. That meant no pay. They had to find other jobs on other ships. "Up and downers" were easily repaired in frame -- that is without removing the whole or even a substantial part of the machinery from the ship. Turbines not so much. Full teardown is needed to get a bad rotor out and tha'ts just for starters. Looking at the state-of-the-art in 1905 when the ships were just pencil marks on paper, I'm sure a lifetime cost/benefit analysis came out in favor of traditional reciprocating steam.
Turbines developed rapidly in both size and reliability. However, had Titanic been an all-turbine ship, it would not have benefited one iota from those rapid improvements. Look at that I-phone you unboxed a year ago. There's a new one now, and yours doesn't have any of the alleged improvements and never will. You're stuck with outmoded technology. Saying that turbines would have been a better choice in 1916 or 1922 or any other particular year misses the point. If built with turbines, the OIympics would have been the same as an MS-DOS computer in a Windows-10 world.
I've often pondered the question, "If Titanic had not foundered and had been brought back to H&W repairs, would the total cost have been greater than scrapping the damaged vessel and applying that money to completing Britannic or starting a 4th Olympic class steamer?" It's a hypothetical question with no answer in reality. History does not reveal its alternatives. But, we all need something to justify that third round of adult pop in the local public house.
-- David G. Brown