What I tried to say is that "discontinued product" is not a fair argument addressed to a person who was just wondering why his device didn't successfully boot up after upgrade.
I'd say that this argument is a very good argument. Discontinued product, specially if a few years passed since it was discontinued, means that vendor won't do much testing (and even less support) of new software releases. So yes, if ROS v7 doesn't work properly on discontinued product, then the fact it's discontinued is a very good argument why v7 doesn't work on this particular product.
As to consequences: as I wrote, it's up to owner to decide, product doesn't have to be thrown in garbage, it's still usable (but not like a shiny new product).
I still believe that it's quite normal for customers to expect successful upgrade to v7 when the product page, despite marking the router as discontinued, offers to download routeros-7.13.1
That's your feelings about it. I say that MT went overboard by actually offering v7 for download for that device architecture and it's up to every user to assess whether they actually want to try to install it or not. Nobody is forcing anybody to upgrade their routers (it's not a legal requirement I believe ... legal requirement is to secure device not to allow any illegal activities and that can be done in many different ways in ROS).
The problem here is that RB751 is the same architecture (MIPSBE) as some still supported devices (such as RB951G which can run ROS v7 without any issues) and installer doesn't check against particular device type. So currently there's no way of preventing v7 from installing on e.g. RB751 but to allow it to install on RB951G.
And no, links on product page are not that relevant, not many users visit product page to fetch upgrade packages. Some will go directly to general mikrotik's download page (where packages are grouped by architecture) or they will go through in-device upgrade procedures.
Go figure.