It's rare than a third-party SaaS can approximate one of these "core sheets" and most of the exceptions have already been explored over the last several decades years.

You have to remember that an SaaS, just like shrink-wrap software, reflects someone else's model of of a process or workflow and the model and implementation evolve per the timeline/agenda of its publisher.

For certain parts of certain workflows, where there's a highly normative and robust industry standard, like invoicing or accounting or inventory tracking, that compromise is worthwhile and we've had both shrink-wrap and SaaS products servicing those needs for a very very long time. We see churn in which application is most popular and what it's interface and pricing look like, but the domains being served have mostly been constant (mostly only growing as new business lines/fashions emerge and mature).

Most of the stuff that remains in a "core sheet" could benefit from the attention of a practiced engineer who could make it more reliable and robust, but almost always reflects that the represented business process is somehow peculiar to the organization. As Access and FoxPro and VBA and Zapier and so many tools have done before, LLM coding assistants and software building tools offer some promise in shaking some of these up by letting orgs convert their "core sheets" to "internal applications".

But that's not an opportunity for SaaS entrepreneurs. It's an opportunity for LLM experts to try to come in and pitch private, bespoke software solutions for a better deal than whatever the Access guy had promised 20 years ago. Because of the long-term maintenance challenges that still plague code that's too LLM-colored, I wouldn't want to be that expert pitching that work, but it's an opportunity for some ambitious folks for sure.