Should I keep using my Dell Precision 3420 or upgrade for Windows 11 and 3D modeling?

The main issue with continuing to use Windows 10 after security support ends is that newly discovered vulnerabilities will no longer be patched. Your applications may continue working, but the risk increases over time. If your modeling software is Blender, it may keep running normally, but the operating system itself is still the concern.

You could consider Linux, or install Windows 11 using a tool that bypasses some of the hardware checks. Windows 10 LTSC or IoT editions may also have longer support, but they are more specialized and may not be the simplest option for a beginner.

For light gaming and modeling, I would prioritize a newer processor, at least 16 GB of RAM, a 1 TB SSD, and a dedicated graphics card. A modern midrange desktop would be a much bigger improvement than adding small upgrades to the Precision 3420.

The current computer is not worthless, but its resale value is probably modest because of its age and lack of a dedicated GPU. You might get a small amount by selling it locally, but a trade-in offer may be even lower. Back up your files before reusing or selling the SSD, and remember that replacing normal hardware such as RAM or storage generally does not create a Windows licensing problem.

Windows 11 can probably run on this machine with an installation workaround, and the i5-6500 is capable of handling basic everyday use. However, it is still an older four-core processor, and the integrated graphics are a significant limitation for gaming and 3D modeling.

You can likely upgrade the RAM and perhaps the processor, but the improvement would be limited compared with buying a newer system. Putting much money into this particular platform probably would not be worthwhile. You could keep the SSD for use as a secondary or system drive in a future computer, assuming it is compatible, but most of the other parts are unlikely to transfer to a modern build.