If the machine is set up using just a parallel port and breakout board, then LinuxCNC will work just fine.
The problem here is that many of us are using the equivalent of TWO parallel ports - very often from a USS (deprecated, noise problems) or preferably an ESS. Rewiring a whole machine from the outputs of the breakout boards is just too much work.

Now, if someone could provide an ethernet engine for LinuxCNC, so we could change the ESS over and boot Linux, then you might see an awful lot of people having a go at it.Swap ESS for the ethernet/Linux board, boot Linux from a DVD, assign the pins to match the existing wiring, and run. I would consider trying it out myself that way. But I am not going to rewire my machine: it is too busy.

Is there such an external engine?

Cheers
Roger