Another point to note is that Airlines using a uniform fleet strategy choose it as part of a wider business plan. Ryanair only operate short haul European city routes on a point to point basis, so they use a single type of aircraft optimised for that. Most train operators will be running a far wider variety of service types though.
The most efficient airlines also have an upgrade policy which rotates older planes out of the fleet while new models are acquired.
This keeps the fleet young and more easily maintainable.
The railway, by contrast, thinks in terms of 30 years or more and often ends up with many sub-groups of the originally single fleet as they are modified over the years.
BR had a policy of dual-sourcing, so that the single Networker design was bought from both ABB and Metro-Cammell, with different components and spares.
The "uniform" class 158s came with different engine types, so did HSTs.
You only have to look at the chaotic way BR entered the dieselisation era to see what problems are caused by too many variations in types of stock.
Privatisation gave us another burst of small, incompatible fleets as the different TOC owning groups exercised their buying power.
So we ended up with problematic fleets like the 175/180, 458 and even 707 fleets.
Roger Ford called it Stanier-Gresley Syndrome.
The costs of all this fall initially on either the TOCs or the owning Roscos, but eventually it's a cost to the DfT and taxpayer.