I gave the armory upgrade as a recent real example, and also said the point stands for building upgrades. Here's a contrived example for you. I'm in the Industrial age. Redoubts are offered at 33% discount. There are 3 of them, and each upgrade costs 7 million. Whoopee, I can kick all three of them off, and save 7 million. I spend 14 million in total for 3 redoubts, and the upgrades are 12 days long. The next week, Mortar Towers are offered at a 50% discount. There are 4 of them, and each upgrade costs 7 million. I now have a difficult choice to make.
If I let the redoubts continue by the time I upgrade all 3 Redoubts and all 4 Mortar Towers I would have spent 42 million (14 for the Redoubts at 33% discount, and 28 for the Mortar Towers at full price). If I cancel the Redoubts, by the time I upgrade all 3 Redoubts and all 4 Mortar Towers I would have spent 42 million (21 for the Redoubts at full price, and 14 for the Mortar Towers at 50% discount, and the 7 for the cancellation fee of half the resources to cancel thee Redoubt upgrades).
If I could pause construction, then I could spend just 28 million availing both discounts to upgrade my entire set of Redoubts and Mortar Towers.
That's a breakeven example so I have nothing to gain by canceling the upgrade, but for the sake of illustrating the point, suppose there were more Mortar Towers say 5, or even 6. (I should have used the 4 Anti-Tank Gun versus the 7 Towers example, but now I'm too far down the rabbithole to change everything). Now clearly there is an advantage to be gained by canceling the Redoubts and upgrading the Mortar Towers instead.
P.S. Of course, we're not assuming built time as a factor since I've upgraded ~50% of 3 Redoubts by the time the Mortar Tower discounts are offered making it less desirable to cancel upgrades for petty resources which can always be looted.
P.P.S. This is not a strategy game like chess or diplomacy where ELO can be used to determine "best player" or "best alliance", because of many things, chiefly the pay2win element. So I'll never be at the top of any best player or best alliance list since I don't spend a penny on this game. I'm losing enough in the form of time as is. This game is fun for me simply because I enjoy progressing through the ages visually with the element of optimization strategy. If the optimization is lost then it becomes simply a progression game like Game of War or Mobile Strike, which are not worthy of my time.