So, have you posted on the Apple forums about its latest OS rendering all of your third party software incompatible?
I'm sure Elements 2018 will continue to work fine for many years on your old computer. But if you choose to have the latest and greatest hardware, you also have to expect additional costs for the software. And to answer your specific question, you are entitled to a discount on the current software because you are an owner of a prior version. At least, that is the case if you purchased from Adobe and/or have the software registered with Adobe. I'm not sure what the policy is if you purchased from the Apple Store.
Unfortunately, Adobe Photoshop Elements 2018 is not supported on macOS Big Sur, nor does it run natively on Apple's new M1 chip. This isn't very surprising as Photoshop Elements 2018 was released over 3 years before Apple released macOS Big Sur. Your loss of confidence in Adobe, should really be a loss of confidence in Apple to be honest.
Your perpetual license for Photoshop Elements means Elements will continue to work indefinitely as long as you don't change the operating system or change your computer. If you do decide to change your operating system or your computer, you need to ask yourself whether all your applications (not just Photoshop Elements) will still continue to work on your proposed new system. If you were a Windows user you would probably find the answer to that question would be "yes" in most cases. Microsoft makes backwards-compatibility a much higher priority than Apple. Apple do things a little differently, and often make design decisions which allow a clean break from the past. Both approaches have their advantages and disadvantages. As a Mac user you have just found out what one of those disadvantages is.
I can confirm though that even if you bought Adobe Photoshop Elements 2018 via the Mac App Store, you are definitely eligible as a previous customer for the upgrade discount on Adobe Photoshop Elements 2021 but only if you buy it direct from Adobe. (You can't get the upgrade discount if you buy it again from the Mac App Store).
and look for the little chat icon at the bottom-right of your screen:
If you click on that icon you'll open up a chat window with Adobe Customer Care. Note that in some European countries this chat facility is only available Monday to Friday during normal office hours, usually 9:00am to 4:45pm. However, if your English is good, go to Adobe's USA website rather than your localized Adobe website, as the chat facility is open 24 hours a day.