Is it possible to replace the welding machine with a light beam? Using mirrors, lenses, and a battery of any power?
Yes, it can be expensive, yes, it can be inefficient, but it is absolutely not important.
Not about laser
 A: That was the basis of a rather known Russian sci-fy novel from 20s: The Garin death ray (at least known in Russia). Notably, the novel discusses the docusing system based in a parabolic mirror, which is mistakenly called hyperbolic (hence the Russian name of the novel is literally The Garin's hyperboloid). Russians physicists inevitably discuss it when making a point about what can be done with a laser but not with mirrors.
Of course, if we could increase the power infinitely, we could always end up melting something. The meaningful question is really about the efficiency of the power use, but oen has to be clear about what the efficiency means in this context: it is the efficiency of the power transmission from its source to the point that we heat, using a ligh beam. The two obvious problems with using non-coherent light source are:

*

*The light ray diverges, i.e., it is in practice impossible to focusit to a tiny spot at significant distance.

*The destructive interference between the incoherent light waves eats most of the power.

