Unfortunately, the buy once and own forever model just isn't really possible in a world where the hardware and operating system those games are played on is changing constantly. Everyone looks back to the good ol' days of Nintendo cartridges, but the reason those games still work is because Nintendo could not update the NES. The NES hardware was the NES hardware and other than minor variations (like the top loader) it stayed the same. The games, and the hardware they're played on, is eternal.
This just isn't how things work anymore. Since the launch of the App Store, Apple has launched over 30 distinctly different devices, 18 major iOS versions, and hundreds of point releases during that time. What has broken things is the expectation that your 99 cent purchase in 2008 should have been supported through 16 years and all of these hardware / software revisions.
If you downloaded Trism on your iPhone 3G in July of 2008 running iPhone OS 2.0, disconnected it from WiFi and cellular, and threw it in your desk drawer, you can still play that same exact version of that game. That is the only way to have a buy once own forever experience. Subscriptions fix that because ongoing revenue allows for these games to continue to be maintained over time instead of just inevitably breaking with an iOS hardware / software update or requiring you to keep your device in this offline time capsule state.