This is where the maxim of open source being 'free as in puppies not free as in beer/lunch' comes from.
I started my career in open source and still strongly believe in FOSS tenets, but enterprises are looking more to de-risk their projects and platforms.
Risk is reduced when you have a vendor you can hold accountable for issues in the product, hence making proprietary solutions (and sometimes open source but wrapped in enterprise support solutions) the only viable choice for a business.
I am sure large companies have that lever: I know of a large multinational company (around 150,000 employees along all contintents) that moved from SuccessFactors to Workday. Those are huge contracts.
Reasons for switching contracts can be numerous, and, from my experience, the level of support and quality of the actual application is rarely one of such reasons.
Yes, custom B2B support contracts that deviate from the terms of support that everyone else gets is not unusual.
For example: a large organization might get a "Microsoft Custom Support Agreement" if they want to get security updates after the date that MS stops providing security updates.
As with any negotiation, if you have leverage (i.e. dollars), you can negotiate.
While I understand and generally agree with your points, I do find the idea of "holding a vendor accountable" to be more of a philosophy than something I've generally witnessed happen in real life. At some point that needs to stop being weighed on the scales when doing "build vs buy" math for any but the largest companies or most attentive vendors.
I started my career in open source and still strongly believe in FOSS tenets, but enterprises are looking more to de-risk their projects and platforms.
Risk is reduced when you have a vendor you can hold accountable for issues in the product, hence making proprietary solutions (and sometimes open source but wrapped in enterprise support solutions) the only viable choice for a business.