To be frank, this is one of the better decisions out of the options available.
- Order a new model of currently manufactured diesel locomotives. GO now has to train their employees on a new locomotive and reduce the uniformity of their current fleet, only to probably retire them anyways later down the line. Not that expensive, but increases complexity and labour.
- Place a massive order of the current diesel locos GO owns to convince the manu to restart manufacturing. So now we have a whole lot more locos than we are probably going to need, and will need to be replaced later down the line... Very expensive.
- Order new electric locomotives. And run them... where exactly? Very expensive.
- Order new battery locomotives. Battery works best with intermittent catenary, and GO has zero kilometres. Very expensive.
- Order new dual-mode electric-diesel locomotives. Frankly, I believe this is the best option for long-term sustainability. Very expensive.
- Buy used locos that match the current locos. Cheap, easy, and stretches the longevity of the system.
If I was in charge, my go-to option would be to look at new EDMUs if i had the cash. If not, buying used locos seem natural to hold the system over until electrification. Granted, the current timeline for electrification is... not good, but unfortunately because of that we are in a permanent state of "Electrification is just around the corner!" (From a rail vehicle lifetime perspective.) major buy in of diesel locos are difficult to justify.
I would also like to argue: This is not "ancient garbage". Trains are incredible ships-of-theseuses where each part is replaced and fixed over time. These things measure their lifetimes in several decades, and the F59PH aren't pushovers. They're not as powerful as the MPs, but a 6 car train hauled by a F59PH should perform similarly to a 12 car train hauled by a MP54AC.