Without the Higgs mechanism we can't have a gauge theory with massive bosons, the reason is very simple, let's take the example of why we can't have a massive electromagnetic photon:
Electromagnetism is described by the group $U(1)$, this means that under a gauge transformation our vector boson $A_{\mu}$ transforms as:
$$A_{\mu} \rightarrow A_{\mu} + \partial_{\mu}f$$
Mass terms for bosons are of the form : $m A_{\mu} A^{\mu}$, but we have that under a gauge transformation this transforms as:
$m A_{\mu} A^{\mu}\rightarrow m A_{\mu} A^{\mu} + 2m A_{\mu}\partial^{\mu}f+ m \partial_{\mu}f\partial^{\mu}f$, i.e. it's not gauge invariant.
For more complex groups, like $SU(2)$, the reason is exactly the same: the mass term is not gauge invariant.
In conclusion it's not possible to have a gauge theory where the vector bosons are massive, because it would break the gauge invariance, for this reason it was not possible to formulate a quantum theory of the weak interaction without the Higgs Mechanism, because since this is a "contact" interaction, we already knew that it would have been mediated by massive boson.