1. You can, and it makes sense for FOSS projects with real support ecosystems, like RedHat, or for freemium FOSS projects, that explicitly sell support.
2. You can still be locked into a single-supplier when you pay for third-party FOSS support. In fact, most third-party FOSS support comes from a single supplier.
3. FOSS that doesn't have a sustainable business model attached to it rarely serves the needs of enterprises. There's FOSS alternatives to Sharepoint, but what's your FOSS alternative to Tableau? Salesforce? Slack? Will you actually save money by using it, compared to just paying a vendor?