Their message is clear, they want you and all of their customers to buy a new product.
They make those "mistakes" by purpose, otherwise no one would ever upgrade their phone.
Look up market share charts, the most commonly used iPhones are 7, 7plus, 8.
There is no way at a company that they don't test new updates on their most used devices.
They knew it will make their old devices unusable, so you can buy the newest one.
So... if you purchase an iPhone you can make sure it will last between 5-7 years, then you can buy the next one.
It means that your iPhone (calculating with 7 years, to be optimistic) at a $1500 price costs you $215 a year, plus the chargers (which last for a few months) and other accessories, subscriptions (eg. iCloud) not included.
They can't make money if your device lasts forever, but I don't blame Apple for that, every company does it nowadays.
Other than that, yes, this is sad. I loved my 7 plus, but I have to buy a new one, which will be probably the newest model.
By the way, how ironic is that they trash your phone with a big software update which makes it useless, than they release their new model in a few weeks? :)