Your thesis seems to confuse economic input and economic output. If a company is producing the same products or services, but doing so with fewer humans and more robots, it has identical economic output. There's no such thing as being economically productive in terms of the value of services consumed. That's being economically consumptive and it is a necessarily evil, not something to promote. When you arbitrarily increase economic consumption, that means nobody else can consume that economic output. Not another productive company or individual, for one, and also not unproductive individual concerns like leisure activity which always gets a little ignored in these discussions as though that weren't the whole point of all this.
If the robots can do the same thing materially better than humans (as they often can today, especially in rote manufacturing tasks, which is the entire reason this has been done), then switching to robots has increased economic output. The possible negative downside you are looking for is it might change the economic distribution, which is exactly what the tax plan proposes to address.
An efficiency limit is, by the very definition of efficiency, an attempt to make the cake smaller; an attempt to reduce economic output. Efficiency in economic production is basically always good (provided you are properly capturing externalities -- the usual counterexample is increasing efficiency by polluting the river etc. which is not good but is also not truly efficient when you take everything into account).
I don't think a per-robot tax is they way to do it but it's strictly better than what you're suggesting, because what you could equivalently do is distribute all the "wages" that would have gone to humans, to some lucky other humans that don't have to do any labour for it whatsoever, in the same proportion as before. Now the economic output is the same/higher and the distribution is the same, but humans get to either do some other economic activity or leisure activities instead of doing the job of a machine. That's better for everybody -- for the rich CEO, for the newly hired kid, for the single mother with a disabled child at home, the consumers down the line; everybody wins (or at least, doesn't lose) in that model. That's almost the same model you proposed, because it's still an efficiency limit, but it doesn't impose the requirement to invest in labour capital it imposes the requirement to invest in humans regardless of labour. Your objection to producing something from nothing is especially bewildering because in economic terms that's what labour is.
I do think that as time goes on, the economy will have to be restructured to accommodate all of this change and get to what right now appears as a utopia, but will inevitably just be the new normal for people born into that world. And I think that's a good thing.
I'm going to make a statement that is controversial: human labour is a bad thing, and therefore job creation is a bad thing although job creation has been generally a tradeoff that was worth it most of the time in living memory. There's two forms of job creation / job destruction: jobs created because of new economic activity, where the job creation is a necessary evil; and jobs created by the broken window fallacy -- totally useless make-work like paying one person to dig a ditch and another person to fill it in; or paying a human to do job that a robot could fulfill at least as well. For a long time, people focused on the first of those and praised job creation there because that was driving massive economic improvements in both the first and third world, for much of the 20th century at least. But some people are so into the cult of job creation that they are supporting the second like the luddites of old.
I would want the tax base to encourage overall economic efficiency -- which means, yes, robots replacing our jobs, but they're still taxing the ultimate beneficiaries using some monotonically nondecreasing income/tax curve (current progressive income tax fits that, so would a flat tax or a consumptive tax; I think a progressive income tax is better but I think any monotonically nondecreasing curve can be argued to be fair), and that's still the right and fair way to do it.