Computers don’t have hard stop generations like consoles. There are hardware generations, but they run the same software as previous ones. As such you get continuous support on a platform that won’t just change every 5-10 years. You’re also not at the mercy of the hardware manufacturer as you are with a traditional console.
If you decide you wanna build a desktop PC and get rid of your Steam Deck in the future your Minecraft world will run there the same as it would your Steam Deck, even across different operating systems (Windows/MacOS/Linux). Minecraft is built on Java, so anything that can run modern Java can run it provided it has sufficient specs.
The Steam Deck runs Minecraft Java Edition extremely well. I cap it at 60 FPS to conserve battery, but it easily pushes 200 FPS for me with a 32x texture pack and some QOL/performance mods. You can also dock it and plug in a mouse and keyboard if you prefer as it’s extremely versatile.