So you had to add code to detect modifications which itself could be bypassed.

Right, which is why DRM schemes aren't typically implemented in a straightforward way. Instead license checks are added to critical program logic so you can't easily skip it, anti-tamper/debug is added to thwart runtime analysis, and on top of all of this the code is obfuscated to thwart analysis even further. You might be eventually be able to figure it out, but it's designed to make it enough of a slog that nobody bothers to work through it all.