[josephcsible]

> But how you can guarantee that everyone that uses your software has really a license?

You can't guarantee that even with DRM, since it's breakable.

> And once, the product is discontinued, they still could release an update disabling the license check.

They could, but what if they don't? It's not like they're legally obligated to escrow their source code in case they don't, or anything like that.