It isn't that simple. One of the implicit tradeoffs you make buying SaaS is that the overall cost of evolution (development and ongoing maintenance) is subsidized across all of the investment resources and customer base of the vendor. With CRM in particular, ecosystem integration is one of the heaviest buildouts there is because each point solution integration can very significantly in complexity and is also where the combinatoric explosion of misbehavior sets in.
When you decide to pull that in house, you are implicitly burdening yourself with the cost of the buildout as well as ongoing maintenance. True, you could probably knock together an okay v1 of CRM yourself inhouse. But are you really going to get it to and maintain production level quality over time at a lower total cost of ownership? I'm skeptical.
The theoretical party you are describing would probably be better served by simply avoiding Salesforce in favor of a next gen CRM that is both more cost effective and easier to customize. In enterprise contexts, even HubSpot is effectively next gen, but there are also products like Attio et al that have a ton of adoption and strong integration ecosystems (albeit not at the Salesforce level).
When you buy enterprise software from a vendor you are buying more than "just software" you are also hiring a company's services. And the inverse is correct as well, when you choose to build it in house, you are implicitly choosing to hire a team internally to resource all of the services you would've expected that vendor to provide.
Certainly, this tradeoff can still make a lot of sense for some companies. The acid test for that, in my opinion, is whether said company could (and would) actually successfully sell the product they build internally on the open market. If the answer to that is "yes", the prospect of turning a cost center into a profit center can potentially bear significant long term ROI to the company.