A way to evaluate drug efficacy that relies less on comparisons of large groups would be a big step, because there are a lot of disorders that are probably multiple mechanisms manifesting in similar ways
Ultimately there is always going to be some trade-off between making a rigorous statement and a generalizeable one, especially because biology seems to have some pretty messy abstractions. As collecting and analyzing large amounts of data becomes more feasible, I don't see why there shouldn't be some efforts to consider genetic (or environmental) heterogeneity in animal models. Ideally I think it'd be cool to approach any given question with parallel methods that can try to address how the hypothesis holds up both in more narrow but well controlled situations, and in the "wild"
Yes we would do fewer experiments since you would need more mice per experiement, but the results of those experiements would be far more robust.