The thing Interslavic is doing has also been tried before, but the illustrative example isn't Esperanto (which is an ad-hoc mixture of words taken arbitrarily from existing languages and put into a synthetic grammar) but rather Interlingua, which is a kind of streamlined Romance language. It too is more-or-less comprehensible by people who speak any Romance language, and was intended to be used in much the same way. In fact, to quote the creator of Interlingua writing
in Interlingua:
Le sol facto que importa (ab le puncto de vista de interlingua mesme) es que
interlingua, gratias a su ambition de reflecter le homogeneitate cultural e ergo
linguistic del occidente, es capace de render servicios tangibile a iste precise
momento del historia del mundo. Il es per su contributiones actual e non per le
promissas de su adherentes que interlingua vole esser judicate.
(Edit: to translate for non-Romance speakers, this means, "The only important fact (from the point of view of Interlingua itself) is that Interlingua, thanks to its goal of reflecting the cultural—and therefore linguistic—homogeneity of the West, is capable of providing tangible services at this precise moment in the history of the world. It is for its actual contributions, and not the promises of its adherents, that Interlingua wishes to be judged.")
It too has met with only limited success: there were some short-lived scientific publications in Interlingua, for example, for the intended audience of all Romance-speaking scientists. Even so, it never attracted much of an audience. That's not to say such a thing could never happen: merely that it's not unreasonable to be pessimistic here.