You've posted in the XPS desktop forum, with what presumably is an XPS 9560 notebook computer.  

If so, it uses a seventh-generation CPU that Windows 11 does not support.  The support decision was Microsoft's, not Dell's.

And interestingly enough, this system was released in 2016, so it's had a 9-year run with OS support.

In contrast, Apple drops support for platforms at the seven year mark, so the platform you "abandoned" had a longer support span than the Macbook you used as its replacement.

The issue isn't with Dell -- no matter what Dell decided to do, the fact is that Microsoft requires an 8th generation CPU for Windows 11 support.  The system you're asking about is a seventh-generation model, which doesn't meet Windows 11 requirements.

The situation is the same with Apple MacOS -- the current release of it (Tahoe) is the last one that will support Intel-based Macs.  Since the newest of those dates from 2023, it means that it's the end of the road for them as far as OS updates as little as two years after purchase.

The march of software support is the same for Mac as it is for Windows.