why can't I upgrade my system after MACOS 10.13.6

Which exact Mac do you have?

Next OS up...

macOS Mojave 10.14.6 - Technical Specifications

General Requirements

OS X 10.8 or later

2GB of memory

12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

Some features require an Apple ID; terms apply.

Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:

MacBook (Early 2015 or newer)

MacBook Air (Mid 2012 or newer)

MacBook Pro (Mid 2012 or newer)

Mac mini (Late 2012 or newer)

iMac (Late 2012 or newer)

iMac Pro (2017)

Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

To upgrade beyond 10.13.6 requires a 2012 or later Mac. If that's what you have, see: How to download macOS - Apple Support

Nope. Obsolescence is never planned. It happens with machine age.

You have not told us, what model of Mac you have so we cannot tell you wether you can, or cannot update to something beyond High Sierra.

Hardware merely determines what it can update to based on actual capacity and possibility of successfully running operating systems that need more CPU and RAM power as time goes by.

Instead using accusatory and conspiratory statements, perhaps you'd like some actual help. Tell us what model of Mac you have, and we can tell you wether you can update further or not.

What model / year Mac do you have exactly?

Wether you can update or not will be dependent on the model / year if it is compatible with anything newer than 10.13.6 High Sierra?

If you need a current web browser, see this tip:

Browsers by Mac operating system - Apple Community

3 redeemed wrote:

So, planned obsolescence?

Obsolescence doesn't need to be planned. It occurs normally because of advances in technology.