That’s an empirical fact, not an assumption. We had a whole thread about this here:
So you just assume something despite having no evidence to base it on. You just have your opinion that it’s “not credible”.
That experiment has already been done. Try this classic experiment:
Also, do you remember we had a thread about this paper?
Duplicate Gene, New Tricks
The fact that functionally new genes appear is clear, but the evolutionary process that allows for a new gain of function is not well understood. Näsvall et al. (p. 384) present the innovation-amplification-divergence model which suggests that after gene duplication, ancestral function is maintained but that the duplicate copies can gain new function that is selected for through the accumulation of mutations or changes in expression. Experimental selection on Salmonella enterica allowed an ancestral gene to evolve new enzymatic function in fewer than 3000 generations.
Apparently the “waiting time” to evolve a new enzyme activity in an already existing enzyme that was incapable of it, was in both cases a few months.
Even so, most enzymes can normally catalyze multiple different reactions already:
https://www.sciencedirect.com/science/article/pii/S0959440X17300982
It is now well accepted that most — and probably all — extant enzymes are, in fact, promiscuous [5, 6 ].
Recent large-scale studies, both computational and experimental, have opened our eyes to the enormous functional diversity among existing enzyme superfamilies, the vastness of ‘promiscuity space,’ and therefore the seemingly limitless potential for future evolutionary innovation. Baier et al. surveyed the functional diversity, as represented by Enzyme Commission (EC) numbers, in five common superfamilies [7•]. Each superfamily contained enzymes from all six of the EC classes (Figure 1a). Furnham et al. went further and used a phylogenetic approach [8] to reconstruct the evolutionary histories of 379 superfamilies from the Class, Architecture, Topology, Homology (CATH) database, and to ask how often a change in EC number was observed over the course of their evolution [9•]. While 81% of the functional changes were within an EC class, every possible change between EC classes was also observed (Figure 1b), with the exception of a change from a ligase (EC class 6) to an isomerase (EC class 5). These bioinformatics studies emphasize that there is little, if anything, that constrains particular catalytic chemistries to particular folds.
Four high-throughput experimental studies (reviewed in detail elsewhere [7•]) have reached a similar conclusion. Dozens of enzymes from within the cytosolic glutathione transferase [10], β-keto acid cleavage enzyme [11], metallo-β-lactamase [12], and haloalkanoate dehalogenase [13••] superfamilies were each tested for activity towards a range of different substrates. In each case, many enzymes were found to have multiple functions in vitro . In the most comprehensive study, 217 members of the haloalkanoate dehalogenase superfamily were expressed, purified, and screened for phosphatase or phosphonatase activity towards 167 substrates (most of which were naturally occurring metabolites). The authors discovered breathtakingly broad substrate specificities. A median of 15.5 substrates were recognized by each enzyme, 50 of the enzymes could utilize 40 or more substrates, and remarkably, one enzyme could utilize 143 [13••].
I really recommend you read that thread again. You are operating under a misapprehension. What seems credible to you is not based on facts.