So don't "upgrade" to iOS 13.
Well, yes and no.
I think security is the primary concern, IMHO. I can understand a company not wanting to offer their latest user experience in perpetuity to end users who bring nothing in return. Five years is an eternity in the tech world, particularly in phones. Being sure the device supported will offer the user an adequate user experience is also a balancing act. Do you hobble the latest release in so much as the features offered to the users with the newest devices who are your primary revenue stream, are not getting the best experience possible?
It may have reached the point where Apple needs to bifurcate the upgrade path for older vs. newer devices. Support the security upgrade side as long as possible, as security of mobile devices is really the next big cyber security threat worldwide, IMHO. This has always been a big selling point for Apple. At the same time, offer the latest user experience to customers with the most up to date devices.
Sorry, but if you're running around with a 5 year old phone & tablet, while I have an iPad pro that's 6 moths old, and an iPhone Xr, you're not paying your freight. It's like expecting to live in a SoHo flat that takes up a whole floor of a building while paying the rent on an apartment in the Philadelphia Badlands.