Umm… It’s about “common” knowledge to all Cubase users that upgrades always are in the last few months of every year. You don’t have to be clairvoyant to have that knowledge. You just could have waited until the release of Cubase 11 to activate your upgrade.
And that’s exactly what I have done. I bought my 10 to 10.5 upgrade last summer for a reduced price, and activated it when Cubase 11 was released. Effectively I have upgraded from 10 to 11 for only €35 (or something like that). The only downside is that I was lagging behind for full year versionwise, and had to wait patiently with with activating my bought upgrade for several months. That’s the price I had to pay for getting something for a reduced price.
To be honest… I do not know of any company that will “compensate” you afterwards if you buy a product that is upgraded at a later time (in this case even several months later). That’s what happens to software and even to hardware. Everybody pays the same upgrade price, and making exceptions for people that bought the stuff and activated it more than a half year ago is a bit too far. You get a grace period as a generous treat, and if you are smart you take advantage of that. But if you don’t do that, you are just treated like any other costumer. In that case I’m afraid you have only yourself to blame.