Comment by pfdietz

Yes, and Lidsky's argument works even if one assumes arbitrarily good plasma performance. He was not basing his argument on magnetic field strength or beta limits, or even any particular device geometry. Assume 100T magnetic fields and beta=1; his argument still applies.

What may be confusing you is that very low power density designs, like ITER, may be even worse than Lidsky's bound. So they could be improved somewhat by better magnets or plasma physics tricks. But once Lidsky's bound is reached, further improvements of that sort are no help.