I wonder how much will businesses tolerate this if promised efficiency gains do not materialize for most of them.

I think you've stumbled on the core issue here. For an unchanging company for which their digital business does not need to evolve very much, having a basic IT department may just work. But as we digitize more and make technology a core competency and critical differentiator, a legacy, inflexible tech stack becomes a burden. It doesn't let you add the features that your customers may want, features which might be critical to a continuing and successful business relationship. Your customers then have similar demands from _their_ customers... all the way back to the primary sector.