> OpEx is much easier to get vs CapX

Not where I work at. IT opex are a recurring cost item with no visible benefits, so are squeezed dry at every annual budget review. Capex are seen as investments into something new and good and their impact on the bottom line is amortized over several years.

Now I'm curious: Sarbanes-Oxley pushed vendors into subscription models? How? What exactly did it change in the cost-benefit picture?

S/O changed the way that revenue was recognized when a vendor sold product and services to implement. Basically, you had to wait until the implementation services were completed to recognize product revenue. Time to complete services could be multi-year and customer dependent.